Page:The American Cyclopædia (1879) Volume XI.djvu/540

 522 MICROSCOPE tirely to be relied upon unless the same speci- men is used in comparisons, on the same stand, and with the same illumination. In the fol- lowing table the usual tests, both for direct and oblique illumination, are given : Objective, inches. Angle of aperture. Direct light. Oblique light. 2 or IX 20 Sections of echinus. " of wood. 1 '/nor'/ 28" 83 80 100 140 170 Pollen grains, &c. Tracheae of insects. Pleurosigma atten- uatum. Pleurosigma strigo- sum. Pleurosigma fasci- ola. Do. do. Pinnularia viridis. Cocconeina lanceo- latuin. Pleurosigma fasci- ola. Hyalodiscus Cali- fornicus. Navicula rliom- boides. Grammatophora subtilissima of Providence. The following table exhibits the lineation of different species of diatomacece which have been employed as tests ; the measurements are those of Messrs. Sullivant and Wormley : Teats. Striae in 1-1000 of an inch. 1 Pleurosigma formosum. 36 diag 2 " strigile. 80 trans. 3 86 " 4 5 " attenuatum. 86 " 88 " 6 . ... " strigosum. 42 diaer. 7 8 9 10 " quadratum. u elongatum. " lacustre. 45 48 " 42 trans. 50 diag 11 ... " fasciola. 56 trans 12 13 Navicula rhomboides. Nitzschia sigmoidea. 70 " 70 " 14 15 1C Colletonema vulgare. Grammatophora subtilissima, Greenport. Do. do. Providence. 72 " 70 to 75 trans. 75 to 80 " said to be 75 " 17 Amphipleura pellucida. 130 J. D. Moller of Wedel, in Holstein, who is well known for his beautiful mountings, furnishes a plate on which are mounted, in a line, the following 20 diatoms, in the order designated by Dr. Grunow : 1, triceratium favus ; 2, pin- nularia nobilis ; 3, navicula lyra,&r. 4, na- vicula lyra ; 5, pinnularia interrupta, var. ; 6, Stauroneis pho&nicenteron ; 7, grammatophora marina (more coarsely marked than Bour- goyne's variety) ; 8, pleurosigma Balticum ; 9, pleurosigma aeuminatum ; 10, Nitzschia ampJiioxys ; 11, pleurosigma angulatum ; 12, grammatophora oceanica subtilissima (marina) ; 13, surirella gemma; 14, Nitzschia sigmoidea ; 15, pleurosigma fasciola, var. ; 16, surirella gemma (longitudinal lines and beads) ; 17, cymatopleura elliptica ; 18, navicula crassiner- vis, frustulia Saxonica ; 19, Nitzschia curvu- la ; 20, amphipleura pellucida. Dr. Wood- ward has photographed surirella gemma, and has obtained the longitudinal strife as "rows of minute hemispherical bosses. The fine trans- verse striffi counted longitudinally at the rate of 72 to the TT5 V?r of an inch. Transversely these lines were resolved into beaded appear- ances, which counted laterally 84 to the TT jV* of an inch." (" American Journal of Science," May, 1871.) But all organic markings are va- riable as to dimensions and visibility, and hence are not comparable except when one and the same object is used in the comparison of the efficiency of two lenses. Mr. Nobert has suc- ceeded in giving comparable test objects to microscopists by ruling on glass plates bands of parallel lines, the distances between the lines of successive bands constantly decreasing. His most recent plate contains 19 bands. The distance between the lines in the first band of a Paris line, and increases by -fa n each to the 19th, in which it is TTr ,^jnr. The lines in the higher orders of bands are as close together as those of the diatoms whose markings are difficult to resolve. M. Schultze resolved the 15th band, and subsequently Dr. H. Frey resolved the 17th, with a Hartnack immersion No. 11. In 1869 Dr. Woodward resolved the 18th and 19th bands with an immersion ^ of Powell and Lealand, and Dr. Edward Curtis photographed them. Dr. Wood- ward says ("American Journal of Science," September, 1869): "The photograph of Dr. Curtis was taken without an eye piece and with such a distance that the immersion T V gave 1,000 diameters. The illumination was by sun- light passed through the ammonio-sulphate of copper, a | objective of 148 angle of aperture being used as the condenser, without diaphragm or stop, and obliquity of light obtained by means of the centring screws of the second- ary stage. . . . Returning now to this immer- sion T V, it may be remarked that the work just done with it has an important bearing on the question of the real limits of microscopic vision. Nobert, in sending me the plate above de- scribed, wrote me that in his opinion the 15th band was the limit of possible microscopic vi- sion. He based his opinion upon Fraunhofer's formula with regard to the spectra of gratings, and upon the known wave length of light undu- lations. Dr. Barnard of Columbia college, New York, after reading Nobert's letter, writes me that. in his opinion Fraunhofer's formula does not apply to the visibility of fine lines when observed with a modern microscope of high power, since the great angle of aperture of the objective permits oblique rays to reach the eye, and Fraunhofer's formula applies only when the eye is perpendicular to the grating. Dr. Barnard is therefore of the opinion that the limit suggested by Nobert has no real exis- tence. In his letter, which I should mention was written before he was aware that I had satisfactorily resolved any of the bands beyond the 15th, he proposed that a trial should be made, to resolve the test plate with monochro- matic light, of colors longer than the violet which I had been using. Accordingly, obtain- ing monochromatic light by a prism on which a ray of sunlight was thrown, I succeeded after some trials in satisfactorily resolving the 19th