ZOOLOGY AND BOTAITY, MICEOSCOPY, ETC. 



559 



Gelatine plates, possessing the two qualities of extreme sensitive- 

 ness and great density after development, are essential for the pro- 

 duction of the finest negatives by the foregoing process. The brands 

 known (in America) as "Beebe," "Eastman Eapid," and " Carbutt's 

 Special " combine these qualities in an eminent degree, and are 

 recommended accordingly. Either ferrous-oxalate or pyro-developer 

 may be used with equal success, but they should be strong and active, 

 as a rapid development is necessary to the best results. If ferrous- 

 oxalate be employed, it should be made quite acid with citric or oxalic 

 acid. 



It having been found by actual work that the chemical and visual 

 foci of the rays from a lamp are almost exactly coincident, there is no 

 need of employing specially corrected objectives with this apparatus. 

 And the following table of exposures with Beck's objectives may be 

 depended upon as an accurate basis for work with the average of 

 objects to be photographed : — 



1^ inch, 2 to 3 minutes. 

 2-3 „ 3 „ 4 „ 

 4-10 „ 7 „ 10 „ 



1-5 iuch, 8 to 12 minutes. 

 1-10 „ 1.5 „ 20 



For opaque objects, illuminated by sunlight, exposures of six to 

 twenty seconds, depending upon the power employed and reflecting 

 qualities of the specimen itself, will generally be found sufficient. 



Haner's PhotomicrograpMc Apparatus.*— This ("fig. 97) is a 

 very simple method, devised by Max Hauer, of combining a camera 

 with the Microscope. The standard which carries the cross-bar D 

 slides vertically in the hollow pillar, and can thus be s'jt (by A) at any 



* Dippel's ' Das MikrosVop,' 1S82, pp. 57G-7 (1 fi.?). 



