1894.] MICROSCOPICAL JOUR"N^AL 351 



complex. Under the microscope its exterior face is covered 

 with a strange net-work of sculpture, indicating molecular 

 changes. The crystals often have foreign bodies enclosed with- 

 in their walls — a bubble of gas or a drop of liquid. These mi- 

 croscopic cavities are often ver}' numerous. One which con- 

 tained carbonic acid was exploded in Edinburgh by being 

 warmed in the mouth. 



MICROSCOPICAL APPARATUS. 



Substage Condenser. — According to Peragallo, the plane 

 mirror suffices for all objectives of an aperture not exceeding N. 

 A. 075 — that is, up to about lin. objective — and this is true if 

 you have plenty of light, but on dull days a condenser is useful 

 with any objective from Hn. upwards. Above iin. a condenser 

 becomes really indispensable, as otherwise you will not get suf- 

 ficient illumination, especially when using daylight. You will 

 waut all the light the condenser will collect on most winter days 

 when working with the l-12in. and upwards. A condenser also 

 enables one to illuminate with very oblique pencils, which con- 

 siderably aids the visibility of certaia fine surface markings. 

 Also in certain cases you want to remove the morphological de- 

 tails as much as possible, so as to leave a nearly pure color pic- 

 ture, as in bacteriology, and for this purpose a condenser of suf- 

 ficient aperture is indispensable. I do not think the condenser 

 improves the definition of an inferior objective. A condenser 

 makes the faults of an objective more evident. But with high- 

 power wide-angled objectives all authorities agree that a con- 

 denser of some kind is absolutely requisite, and also that there 

 should be some correspondence bet'veen the angle of the objec- 

 tive and that of the condenser, though what precisely that cor- 

 respondence should be authorities do not quite agree. The 

 only utilisable portion of the cone of light from the condenser 

 is the aplanatic portion. This in conlenser N. A. 1'2 — is about 

 N. A. 05, amply sufficient for any objective up to a l-12in. It 

 is the fashion now amongst the ultra-refined microscopists to 

 speak slightingly of these chromatic condensers of Abbe ; but 

 excellent work has been done, and is being doae, with tiiem. 



Zeiss does not admit that achromatism of the condenser is es- 

 sential to correct microscopical vision, though better for photo- 



