INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 877 



tions are entirely changed, and tliat " aperture " is not a question of 

 " angles" simply as angles, at all. 



Diameter of Microscope-tubes.^ — In a paper read two or three 

 years ago to the Sau Francisco Microscopical Society (now first pub- 

 lished *), Dr. J. H. Wythe says that the diameter of the Microscope- 

 tube has an important relation to the distinctness and luminosity of 

 the image. Few tubes are wide enough to utilize more than a small 

 proportion of the rays proceeding from an objective. The field-glass 

 of the eye-piece should be of the greatest diameter possible for its 

 focal length, and the tube wide enough to receive it, in order to 

 concentrate the greatest number of rays from the objective. The 

 short tubes of French and German Microscopes are supplied with 

 narrow eye-pieces, which cut the cone of rays nearer the objective, 

 and give a more brilliant image than would be possible in a longer 

 tube. If the tube be longer, it must also be wider, and the eye-piece 

 of corresponding diameter. 



Wythe's Amplifiers. — Dr. Wythe in the same paper proceeds to 

 explain his views as to the construction of amplifiers, some of which 

 he exhibited to the Society, which were considered to be a great 

 improvement upon any previously seen. 



" In considering the construction of the Microscope with a view to 

 greater amplification by the eye-piece, it occurred to me that the 

 concave lens or meniscus used to diverge the rays of the objective 

 should form a part of the eye-piece, and be of as large diameter as the 

 tube will allow. If it be of small diameter, it must be placed nearer 

 the objective. This is the form and position of the amplifiers of 

 Tolles, Zentmayer, and others. 



One of the amplifiers, exhibited by me to the Society on a 

 previous occasion, consists of a conical meniscus, whose position in 

 the tube and eficcts correspond with the amplifiers above named. With 

 this simple addition placed in the lower end of the draw-tube the 

 magnifying power of an objective can be nearly doubled with little 

 loss of light or of definition. 



The other form of amplifier now exhibited is still better. A 

 double concave lens, or meniscus, of as great diameter as the tube 

 will allow and of considerable diverging power, is placed at a distance 

 of from 2 to 4 inches in front of the eye-piece. In the improved 

 form in wliich I now present it, a concave meniscus of 6 inches 

 equivalent focus and 1;\- inch diameter (which formerly served as 

 part of the object-glass of a small telescope), is placed in a draw- 

 tube at the end next the eyc-pieco and about 3 inches from the 

 latter. To counteract the aberration of the amplifier, I have some- 

 times substituted foi' the plano-convex field-glass of the Huyghenian 

 eye-piece a convex meniscus of short focus, which gives also a very 

 wide and flat field of view. Ordinary eye-pieces and the pcrisco])iC 

 eye-pieces of Gundlach may also bo used with the amplifier. Tlie 

 amplifying eyc-pieco, thus constructed, 1ms given me great satisfaction. 

 If the concave meniscus were made achromatic, it would doubtless be 



* ' Am. Joiini. Mior.,' v. (1880) p. 81. 



