Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/286

Rh 270 MICROSCOPE the object at an obliquity corresponding to that at which the most divergent rays enter the objective. Now, although in the case of objects whose markings are only superficial such obliquity may not be productive of false appearances (though even this is scarcely con ceivable), it must have that effect when the object is thick enough to have an internal structure ; and the experience of all biological observers who have carried out the most delicate and difficult investigations is in accord, not only as to the advantage of direct illumination, but as to the deceptiveness of the appearances given by oblique, and the consequent danger of error in any inferences drawn from the latter. Thus, for example, the admirable researches of Strassburger, Fleming, Klein, and others upon the changes which take place in cell-nuclei during their subdivision can only be followed and verified (as the writer can personally testify) by examination of these objects under axial illumination, with objec tives of an angle so moderate as to possess focal depth enough to follow the wonderful differentiation of component parts brought out by staining processes through their whole thickness. The most perfect objectives for the ordinary purposes of scientific research, therefore, will be obviously those which combine exact definition and flatness of field with the widest aperture that can be given without an inconvenient reduction of working distance and loss of the degree of focal depth suitable to the work on which they are respectively to be employed. These last attributes are especially needed in the study of living and moving objects ; and, in the case of these, dry objectives are decidedly preferable to immersion, since the shifting of the slide which is requisite to enable the move ment of the object to be followed is very apt to produce disarrange ment of the interposed drop. And, owing to the solvent power which the essential oils employed for homogeneous immersion have for the ordinary cements and varnishes, such care is necessary in the use of objectives constructed to work with them as can only be given when the observer desires to make a very minute and critical examination of a securely-mounted object. The following table expresses the magnifying powers of objectives constructed on the English scale of inches and parts of an inch, with the 10 inch body and the A and B eye-pieces usually supplied by English makers, and also specifies the angle of aperture which, in the writer s judgment, is most suitable for each. He has the satisfac tion of finding that his opinions on this latter point, which are based on long experience in the microscopic study of a wider range of animal and vegetable objects than has fallen within the purview of most of his contemporaries, are in accordance with the conclu sions drawn by Professor Abbe from his profound investigations into the theory of microscopic vision, 1 which have been carried into practical accomplishment in the excellent productions of Mr Zeiss. Magnifying Magnifying Focal Angular Power. Focal Angular Power. Length. Aperture. A Eye B Eye Length. Aperture. A Eye B Eye piece. piece. piece. piece. 4 Incl es 9 12 18 1 inch. 50-80 200 300 3 12 18 27 i 95 250 375 2 15 25 37 ft 110 300 450 li 20 36 54 i 140 400 600 1 30 50 75 A 150 500 750 s 40 75 112 A 160 600 900 I 45 100 150 A 170 800 1200 A 70 125 187 For ordinary biological work, the |, T V, and T objectives, with angles of from 100 to 120, will be found to answer extremely well if constructed on the water-immersion system. Each of these powers should be tested upon objects most suited to determine its capacity for the particular kind of work on which it is to be employed ; and, in such testing, the application of deeper eye pieces than can be habitually employed with advantage will often serve to bring out marked differences between two objectives which seem to work almost equally well under those ordinarily used, defects in definition or colour-correction, and want of light, which might otherwise have escaped notice, being thus made apparent. No single object is of such general utility for these purposes as a large well-marked Podura scale ; for the eye which has been trained to the use of a particular specimen of it will soon learn to recognize by its means the qualities of any objective between 1 inch and J inch focus ; and it may be safely asserted that the objective which most clearly and sharply exhibits its characteristic markings is the best for the ordinary work of the histologist. For the special attribute of resolving power, on the other hand, tests of an entirely different order are required ; and these are fur nished, as already stated, either by the more &quot;difficult&quot; diatoms, or by the highest numbers of Nobert s ruled test-plate. The diatom-valve at present most in use as a test for resolving power is the Amphipleiira pcllucida, the lines on which were long supposed to be more closely approximated than those of Nobert s 1 See his paper on &quot; The Relation of Aperture and Power iu the Microscope,&quot; in Jour. Hoy. Micros. Soc., 1882, pp. .-iOO, 4fiO. nineteenth band, being affirmed by Mr Sollitt to range from 120 to 130 in Toinr of an inch. But the admirable photographs of this valve obtained by Colonel Dr Woodward have confirmed the conclusion long previously expressed by the writer, that this esti mate was far too high, being based on the &quot;spurious lineation&quot; produced by diffraction, and show that the striae on the largest valves do not exceed 91, while those on the smallest are never more numerous than 100, in ^^ of an inch. The same admirable manipulator has also obtained excellent photographs of another very difficult test-diatom, Siirirella gemma, from which it appears that its transverse striae count longitudinally at the rate of 72,000 to the inch, whilst the beaded appearances into which these may be resolved count transversely at the rate of 84,000 to the inch. Thus it appears that the complete resolution of these &quot;vexatious&quot; diatoms does not require by any means the maximum of aperture, but is probably dependent at least as much on the perfection of the corrections and the effectiveness of the illumination. It must be understood that there is no intention in these remarks to undervalue the efforts which have been perseveringly made by the ablest constructors of microscopic objectives in the direction of enlargement of aperture. For these efforts, besides increasing the resolving power of the instrument, have done the great service of producing a vast improvement in the quality of those objectives of moderate aperture which are most valuable to the scientific biolo gist ; and the microscopist who wishes his armamentum to be com plete will provide himself with objectives of those different qualities, as well as different powers, which shall best suit his particular requirements. 2 ILLUMINATING APPARATUS. Every improvement in the optical performance of the compound achromatic microscope has called forth a corresponding improve ment in the illumination of the objects viewed by it, since it soon came to be apparent that without such improvement the full ad vantage of the increased defining and resolving powers of the objectives could not be obtained. For the illumination of trans parent objects examined by light transmitted through them under low powers of moderate angle a converging pencil of rays reflected upon their under surface by a concave mirror is generally sufficient, a &quot;condenser &quot; being only needed when the imperfect transparency of the object requires the transmission of more light through it. And the microscopist engaged in ordinary biological studies, who works on very transparent objects with objectives of ^ or ^ inch focus, or -fa inch immersion, will find that the small concave mirror of short focus with which the Continental models are fur nished (see fig. 28) will generally prove sufficient for his needs. This mirror is usually hung at such a distance beneath the stage that parallel rays falling on it are brought to a focus in the object as it lies on a slip of glass resting on the stage ; and thus, when the instrument is used by day, the light of a bright cloud (which is pre ferable to any other) gives a well-illuminated field, even with the powers last-mentioned. But when lamplight is need its divergent rays are not brought to a focus in the object by a mirror that is fixed as just stated ; and the distance of the mirror beneath the stage should be made capable of increase (which is easily done by attaching it to a lengthening bar), so as to obtain the requisite focal convergence. Still the best effects of objectives of less than % inch focus cannot be secured without the aid of an achromatic condenser, interposed between the mirror and the object, so as to bring a larger body of rays to a more exact convergence. When objectives of still higher power are employed, the employ ment of such a condenser becomes indispensable ; and when the highest powers are being used by lamplight, it is desirable to dis pense with the mirror altogether, and to place the flame exactly in the optic axis of the microscope. The condenser should be an achromatic combination, corrected for the ordinary thickness of the glass slip on which the object lies, and capable of being so adjusted as to focus the illuminating pencil in the object. As it is often found desirable that an object should be illuminated by central rays alone, or that the quantity of light transmitted through it should be reduced (for bringing into view delicate details of structure which are invisible when the object is flooded with light), every microscope should be provided with some means of cutting off the outer rays of the illuminating cone. The &quot;dia phragm-plate &quot; ordinarily used for this purpose is a disk of black metal, pivoted to the under side of the stage, and perforated with a graduated series of apertures of different diameters, any one of which can be brought, by the rotation of the disk, exactly into the optic axis of the microscope. But the required effect can be much more advantageously obtained by the &quot;iris-diaphragm,&quot; in which a number of converging plates of metal are made so to slide over each other by the motion of a lever or screw that the aper ture is either enlarged or diminished, while always remaining prac tically circular as well as central ; and in this manner a continuous 2 See the remarks of Mr Dallinger, whose experience in the application of the highest powers to the study of the minutest living objects is probably greater than that of any living observer, in Jour. Roy. Micros. Soc., December 1882, p. 853.