ILLUMINATION. 185 



ticular form of condenser would be the best adapted for 

 the purpose. 



In determining how far it would be practicable to cut 

 down the angle of the condenser, thus reducing the 

 illuminating cone of light, we have made countless ex- 

 periments, while the IOAV stage of the Histological ren- 

 dered it imperative that the focal length of the lens 

 should be such as would best accommodate the little 

 stand. To sum up all these trials, we find that the 

 cheapest inch objective mac!e by Mr. G-uncllach, or the 

 inch of the Messrs. Beck's " National Series," are, either 

 of them, well adapted for the purpose. Mr. Gundlach's 

 inch has a rubber front which can be removed, while 

 the setting of the Beck " National " is extremely short, 

 and thus suited in this respect for the purpose. 



This, then, is the author's arrangement for work with 

 low or moderate powers by daylight illumination, and 

 the condenser described has become almost a fixture. 

 In the darkest clays there will be plenty of light, using 

 the concave mirror, while in bright, sunny days the 

 plane can be substituted. The general amount of illu- 

 mination can be changed at will by merely raising or 

 lowering the sub-stage, and the nicest effects in the way 

 of definition obtained. The swing-bar can also be 

 placed so as to afford central illumination, or it may 

 (condenser and all) be swung laterally up, say to an 

 angle of 40 or 50 degrees from the axis; and it further 

 remains to say that either of these cheap objectives are 

 real good, honest glasses for the money. 



