134 



THE AMERICAN MONTHLY 



[July, 



the benzole is supposed to make cer- 

 tain colors more permanent. 



To avoid the possible separation 

 of the cover-glass from old mounts 

 some preparers finish their slides with 

 a ring of damar or balsam applied on 

 the turn-table. This makes a neat 

 finish, but when done it is impossible 

 to tell in what medium an object is 

 mounted. It seems better, therefore, 

 to leave all balsam mounts without 

 finishing rings, as a distinguishing 

 characteristic. 



The Selection and Use of Micro- 

 scopical Apparatus. 



We take the following account of 

 a demonstration by Mr. E. M. Nelson 

 before the Quekett Club (London) 

 from the English Mechanic. So 

 long as the author does not go too 

 deeply into theoretical discussions of 

 the microscope his contributions to 

 practical microscopy are worthy of 

 careful reading, since he is a prac- 

 ticed and very expert manipulator, 

 having every facility for experiment- 

 ing, and leisure to devote to it, 



' Commencing with the choice of a 

 microscope stand Mr. Nelson said 

 the different forms of stands might 

 be broadly divided into two classes : 

 (i) those in which the optical body 

 was supported on a bar or arm pro- 

 jecting at right angles from the pillar- 

 rack, as in Powell and Lealand's, 

 and (2) those known as the Jackson- 

 Lister, in which the optical body had 

 the rackwork applied directly to it 

 and supported by a limb, (two such 

 microscopes, by Swift and Son, were 

 shown as examples.) His own ex- 

 perience was decidedly in favor of 

 the Powell form, as lending itself so 

 perfectly to the application of the best 

 system of fine adjustment that he 

 knew of — the long lever acted upon by 

 a fine screw. Moreover, this form, 

 when properly made, allowed the 

 complete rotation of the stage, and 

 plenty of space for manipulations on 

 the stage. The Jackson-Lister might 

 possess an element of steadiness be- 



yond the Powell for use on board 

 ship, (as stated by Dr. Carpenter,) 

 but he could not regard that point as 

 very essential, and it certainly did not 

 compensate for the disadvantages 

 which he feared were insuperable in 

 that model. The application of a 

 good fine adjustment to the Jackson- 

 Lister presented great difficulties. He 

 might at once state his conviction that 

 the fine adjustment attached to the 

 body-tube — the short lever acted upon 

 by a screw — was "radically bad." 

 He considered Messrs. Swift had hit 

 upon a system of fine adjustment for 

 the Jackson-Lister that was, at any 

 rate, far better than the short lever ; 

 they had applied a long lever verti- 

 cally behind the body-tube, acted 

 upon by a screw at the side, and thus 

 attained an approximation to the cer- 

 tainty and delicacy of the Powell fine 

 adjustment. The Jackson -Lister 

 ought to be a far less costly instru- 

 ment than the Powell, as machine- 

 work could be employed largely in 

 its manufacture. For the most diffi- 

 cult class of work he gave the prefer- 

 ence, without reserve, to the Powell 

 model ; it must, however, be thor- 

 oughly well made, and must neces- 

 sarily be a costly stand. He could 

 not regard any microscope as worthy 

 to be called a scientific instrument 

 unless it were provided with a cen- 

 tring substage, and, indeed, he must 

 emphatically urge that scientific mi- 

 croscopy really began with the use of 

 a substage condenser — of which he 

 would speak later on. Regarding 

 the choice of objectives, they should 

 be selected so that the battery might 

 be increased without having to ex- 

 change, and without useless expendi- 

 ture. The beginner might have a i^ 

 inch and a f ; later on a i might be 

 added, and as a higher power a -^ 

 oil-immersion of 1-43 n. a., such as 

 Mr. Powell made so successfully. 

 For all v/orking purposes the battery 

 would then be complete, and the mi- 

 croscopist equipped to repeat any re- 

 sults hitherto obtained. As luxuries 

 a 3 inch, ^, and -^^ might be got. It 



