Page:The American Cyclopædia (1879) Volume XI.djvu/535

 MICROSCOPE 517 the American and English opticians, and more recently by the French and German. The English uniformly, if we except the amateur efforts of Mr. Wenham, make the front set movable; the American opticians generally move the two posterior sets, the front being immovable ; the. latter method is better, the object being kept easier in view during the adjustment, and there being no danger of bringing the front lens in contact with the object. The objectives of Mr. Ross have al- ways maintained a high character, and have been more expensive than those of Powell and Lealand, or Smith, Beck, and Beck, though they probably do not surpass them. The busi- ness is now conducted by Mr. Thomas Ross and Mr. Wenham. Mr. Ross's objectives range from 3 in. to T ^ inch. Messrs. Powell and Lealand, so far at least as the objectives and accessories are concerned, are quite equal to any living opticians. Both the low and high powers of this firm are of the finest character. "While the general plan is the same, there are some peculiarities in their high power objec- tives worthy of notice. The front set is triple, and the front lens, being of crown glass, is less liable to injury from wiping, or accidental con- tact with the object, than the soft flint of oth- er opticians ; but as it is a very thin plano- convex, merely cemented to the concave, and not burnished in, it is liable to injury by part- ing the cement. Messrs. Smith, Beck, and Beck (now R. and J. Beck) are more widely known in this country' than the other firms. Their " students' " and " educational microscope " are the forms usually sold by the dealers. As opticians, for the lower power objectives, they stand side by side with the others named. They did not until lately furnish objectives higher than -J-, of 125 angle. We can only mention the names of Fachet, Oberhauser, and Kellner, whose objectives are excellent, but inferior to the American or English ; they are, however, much cheaper, and the mechanical work upon their instruments is very excellent. The name of Prof. Amici of Modena was long associated with the microscope. The objec- tives made by him consisted of six series, with angles varying from 26 to 160; the higher powers had no adjustment for cover, but the front lens was slightly concave, and a drop of water introduced between the cover and the objective, thus in a measure rendering the ad- justment unnecessary. The marked advan- tage in the plan of Prof. Amici, of introducing a stratum of water between the front lens of the object glass and the thin glass covering the object, was fully shown by its originator 12 or 14 years ago ; but it was only after Nachet of Paris adopted the idea and thereby greatly improved the working of his objectives, that opticians viewed Amici's principle with favor. E. F. Hartnack of Paris, the successor of Ober- hauser, at once adopted the u immersion " system, as it is called, and soon came to the very front of all objective makers. The ad- vantage of this system is well stated by Har- ting : " As the water is a stronger light-re- fracting medium than air, the reflection of the rays of light is much diminished at the under surface of the objective; indeed it al- most entirely ceases. Hence more rays of light pass into the microscope, and the thin stra- tum of water has nearly the same effect as an enlargement of the angle of aperture. This favorable modification influences chiefly the pe- ripheral rays, which fall most obliquely. The peripheral rays have most influence on the for- mation of the image, which takes place in front of the eye piece ; and as, by their passing through a transparent object, they are for the most part deflected from their course, and the slight deviations thus caused become visible in the image, the defining power of the microscope must necessarily be increased by the stratum of water." The magnifying power as well as the angle of aperture is increased by the stratum of water in which the objective is immersed, for the water acts like the cover glass, and the lenses must approach each other in proportion to its thickness. The advantage of an immer- sion lens over the ordinary dry objectives can- not be better shown than by the following ex- perience of Dr. J. J. Woodward, United States army. He had three superior lenses, all made by Powell and Lealand: a -^ and a 7 V dry lenses, and a ^ immersion lens. The former lenses could resolve only the 15th band on Robert's 19-band test plate, but the T ^ immer- sion lens not only resolved into lines this 15th band, but also the 16th, 17th, 18th, and 19th bands. Finally, we must not omit to name Mr. Lister, who first pointed out some pecu- liarities possessed by a combination of three achromatics with their plane sides toward the object, the crown and flint being cemented together, and which were the basis of sub- sequent improvements. The peculiar form adopted by Lister has long since been aban- doned ; but the principles are the same. The stands furnished by the principal makers ex- hibit a great variety of patterns, and combine various excellences. The most desirable points, viz., freedom from tremor, ease of illumina- tion, particularly oblique illumination, facility in the application of the accessory apparatus, and delicacy of adjustment, are nearly equal in the first-class stands of the principal opti- cians ; but perhaps, on the whole, the best are those of Mr. Zentmayer and of R. and J. Beck. Their large stands mounted on two pillars are remarkably steady. The stage, thin but sub- stantial, is large, and its movements are very smooth and delicate. The illuminating and accessory apparatus is carried by a sub-stage below, and most easily applied. The illumi- nating mirror is large, and so mounted as to give very oblique illumination. The rackwork of the quick adjustment is always of the smoothest and best character in all of the in- struments made by these firms, and the slow adjustment very delicate. The greatest fault