1884.] 



MICROSCOPICAL JOURNAL. 



147 



mination. For opaque objects he 

 thought nothing had been devised so 

 good as Lieberkuhns, and objects 

 ought as far as practicable to be 

 mounted for use with Lieberkuhns, 

 and not covered up with paper. If 

 the side illuminator were used it 

 should be attached to a fixed part of 

 the stand, not to the body-tube or 

 stage. The vertical illuminator was 

 a difficult apparatus to manipulate, 

 and required great patience. It af- 

 forded a remarkable proof of the ex- 

 istence of aperture beyond the dry- 

 lens limit. He exhibited a diagram 

 of the back lens of a j\ oil immersion 

 objective of i -43 n. a. when used 

 with the vertical illuminator ; the 

 outer zone represented the aperture 

 beyond i-o n. a. The first require- 

 ment with the vertical illuminator 

 was to find an object in close adhe- 

 rence to the cover-glass, for it was 

 only such an object that lent itself to 

 that method of illumination. He was 

 afraid he could not say anything use- 

 ful on that matter unless supported 

 by the microscope ; but he had re- 

 peatedly exhibited the apparatus at 

 the club, and therefore he would not 

 then dwell further upon it. As a 

 camera lucida he had been satisfied 

 with Beale's neutral tint i-eflector. 

 On the use of the eye-piece screw- 

 micrometer he said that for delicate 

 measurements it should be mounted 

 on a separate stand not to touch the 

 microscope ; in that w^ay he had re- 

 peatedly counted 90 — 100 lines in the 

 whole space of an inch. As to polar- 

 isation, he had so little time left that 

 he must be content with the simple 

 recommendation of polarisers with 

 large field. He might, however, re- 

 mark that many minute objects gave 

 curious ertects of polarisation — for in- 

 stance, the Podura scale — and many 

 of these yielded most beautiful pic- 

 tures when viewed with polarised 

 light. He then referred to the dif- 

 fraction appearances presented at the 

 back of objectives in viewing A. pel- 

 lucida by axial light and again by 

 oblique light, using, in both cases. 



dry and immersion condensers. He 

 also showed by diagrams the pro- 

 gressive interpretations that had been 

 made of the structure of P. angula- 

 tum and P. J'or?nosum during the 

 past 50 years, and concluded by some 

 general observations on the difficulties 

 of interpreting the images presented 

 by microscopic objects requiring high 

 magnification.' 



Microscopical Technic. 



Mounting in Fluids. — We would 

 not care to undertake the task of enu- 

 merating the numerous fluid concoc- 

 tions that have been recommended 

 from time to time for mounting mi- 

 croscopical specimens. Most of them 

 are good enough in their way, but 

 possess no special merit whatever, 

 and are no better than those men- 

 tioned in every book on mounting. 

 A reference to page 5 1 will indicate 

 by what rule the liquid to be used 

 should be selected. For. very deli- 

 cate and finely marked specimens — 

 such as grains of starch, for example, 

 in which it is desired to show the con- 

 centric markings as clearly as possi- 

 ble — water is unquestionably the best 

 medium. For other specimens which 

 are less transparent, and for injected 

 or stained histological specimens, gly- 

 cerin is to be recommended, and by 

 mixing water and glycerin in various 

 proportions media can be prepared 

 suited to any specimen. Further on 

 will be given the composition of a 

 number of fluids for mounting that 

 have been proposed by mounters of 

 experience. At this place it may be 

 said that as a rule water, glycerin, 

 and mixtures of these, will answer 

 all ordinary requirements. It is usual 

 to employ camphor water, which is 

 water that has been kept in a bottle 

 with a piece of gum camphor, or else 

 water containing a trace of carbolic 

 or salicylic acid or thymol. The ob- 

 ject of such additions is to prevent 

 fungoid growths in the fluid after 

 mounting. We have entirely dis- 

 carded the use of such agents in 



