INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 509 



in such a manner tliat the brown liquid covers them completely ; tlien 

 rinses them under the tap, and stands them on edge on several thick- 

 nesses of blotting paper. When dry, they are chemically clean and 

 require no further rubbing. Before mounting on them, the dust 

 should be brushed off" with a camel's haii- brush. 



With new covers, an ounce or so of them are placed in a wide- 

 mouthed vial, and covered with the cleaning solution, shaking them 

 up occasionally, so that they become separated. After three or four 

 hours, pour off the acid solution and wash the covers in the bottle 

 with water, pouring it on and off until the water remains colourless, 

 after continued shaking of the bottle. When wanted for mounting, a 

 cover can be taken out of the bottle with a pair of forceps, and 

 wiped dry with a linen rag. Cleaning covers in this way gets them 

 thoroughly clean, and breaks very few of even the thinnest. 



When it is desired to clean off specimens which have been 

 mounted, either in balsam or in a watery medium, so as to save both 

 the cover and the slide, slightly warm the specimen over a spirit 

 lamp, and push the cover with a pair of forceps into a vessel contain- 

 ing alcohol and hydrochloric acid in equal proportions. After having 

 scraped off the balsam or cement with a knife, drop the slides into 

 the bichromate of potash solution, in which they nuist remain several 

 days, when they can be washed under the tap and dried on blotting 

 paper like new ones. The covers also should be left a few days in 

 the acid solution, and are then transferred to the bichromate of potash 

 solution, and treated like new ones. 



Conditions of Aplanatism of Systems of Lenses.* — Prof. Abbe 

 publishes an interesting paper under this title. 



As hitherto defined, aplanatism is simply the " elimination of 

 spherical aberration for a pair of coujugate points on the axis ; " the 

 word, however, as practically applied is used with a wider meaning, 

 denoting the capacity of a system of lenses to produce a well- 

 defined imago of an object by a cone of rays of appreciable (i. e. not 

 infinitesimal) divergence ; and by object is meant, not a puint on the 

 axis, but a surface perpendicular to the axis. 



On analyzing the conditions under which an imago is produced 

 with largo angles of divergence, it ai)pears that even when spherical 

 aberration is perfectly corrected for the axis, images of unequal 

 linear amplification may bo formed by the various portions of the 

 avaihible ajicrture of the lens-system. The imago ot an axial element 

 (or minute portion) of an object, which is formed by a pencil of rays 

 inclined to the axis (tlirough any excentric part of the aperture), has 

 u different linear amplification to that which an ini;igo has which 

 originates simultaneously from the rays nearer tho axis (through the 

 central part of the aperture). In fact, tho imago produced by a 

 cone of rays of wide ajterturo is tho result of the 8U})erposition 

 of tlio innumerable images which tho various elements of tho free 

 aperture would produce smijJy, and these may bo shown isolated by 

 the aid of diaphi*agms. If the linear amplification of those " partial " 



• 'SB. Jtn. Ocstll. M.d. mid Ntitiirw.,' 1871), p. 129. Ucviseil l.y Profi'dsor 

 Al'bo, with iliu}jniin8 (Figa. 35, :tt3, iiud 'SI). 



