1881.] 



MICKOSCOPICAL JOUEJNAL. 



195 



this comprises the whole history of 

 the use and improvement of the com- 

 pound microscope in this country ; 

 and it is not easy to realize that all 

 this is embraced within the recollec- 

 tion of men now living. 



Monochromatic Light for Re- 

 solutions. — Some time ago M. Eu- 

 gene Mauler addressed a communi- 

 cation to the Societe Beige de Micros- 

 copies concerning the use of a blue 

 cover-glass for mounting diatoms, 

 claiming that it facilitated the reso- 

 lution of tests. We translate the 

 following from the Bulletin : — " In 

 order to obtain monochromatic light 

 to facilitate the resolution of difficult 

 tests, M. Mauler employs a blue co- 

 ver ; this method, simple as it is prac- 

 ticable, is destined to render good 

 service to diatomists. The assembly 

 was convinced of the value of the 

 plan ; a trial made with a y^^-inch 

 homogeneous immersion objective of 

 Zeiss yielded very good results — the 

 resolution of Surrirella gemma was 

 easily accomplished." 



In another letter M. Mauler wrote : 

 — "According to the more or less 

 perfect quality of the objective, there 

 is more or less advantage in using 

 these blue covers ; thus with an ob- 

 jective No. 5 of Seibert, which gives 

 me a certain amount of color, the re- 

 sult is surprising, but with an objec- 

 tive No. 7 by Hartnack, the effect is 

 less noticeable, because that objective 

 is very well corrected." 



It would seem, therefore, that the 

 advantage of the blue cover-glass de- 

 pends upon the correction of the ob- 

 jective, and not merely upon the 

 color of the light it transmits. 



M. Mauler has kindly presented to 

 us a slide of Grammatophora ma- 

 rina mounted under a blue cover, 

 and while we hesitate to express any 

 decided opinion as to the advantage 

 to be derived from the blue glass 

 without further trial and comparison, 

 we can say that the light is very 

 agreeably modified, and the stri^ of 



the diatoms are very clearly defined 

 with an inferior objective. 



Accessories.— Last month we pro- 

 mised some observations about ac- 

 cessory apparatus for the microscope. 

 Just as there is a tendency to pur- 

 chase large and expensive stands, 

 there is an idea that a large number 

 of accessories is requisite for micro- 

 scopical work. But this is by no 

 means true. However, there are a 

 few which are really useful, and it is 

 about some of these we intend to 

 write. 



Not many years ago a costly achro- 

 matic condenser was thought to be a 

 very valuable piece of apparatus. To- 

 day such condensers are seldom used, 

 and they are quite unnecessary. For 

 giving intense illumination for high 

 magnification they are not so good as 

 much cheaper appliances, and for 

 oblique light the more simple forms 

 of immersion condensers are pre- 

 ferred. 



For routine work with medium 

 powers (up to a ^-inch for example) 

 no condenser is necessary, but, nev- 

 ertheless, a simple form of con- 

 denser with a low angle and provid- 

 ed with suitable stops is very useful. 

 In our opinion the Webster con- 

 denser is an excellent form. It has 

 been our custom to keep a Webster 

 condenser constantly on the stand. 

 It affords a ready means of regulat- 

 ing the illumination as different ob- 

 jectives are employed, or when the 

 camera-lucida is to be used, and it is 

 an excellent substitute for a parabo- 

 loid, although the dark-ground ef- 

 fects obtained are not quite equal to 

 those of the paraboloid. 



For intense illumination, such as is 

 required when very high-power 

 lenses and deep eye-pieces are used, 

 probably an ocular with an achro- 

 matic eye-lens, such as a Kelner 

 eye-piece is preferable to anything 

 else. 



A polariscope is certainly useful, 

 but it is more used for fine displays 



