272 HIGH POWER MICRO-PROJECTION [Cn. IX 



substage condensers, and without oculars; but they realize that 

 in laboratory work and for some special lectures to small classes it 

 is of the highest advantage to be able to show pictures of photo- 

 graphic sharpness in all details. For this it is necessary to use, first 

 of all, a substage condenser which will give a light cone of sufficient 

 aperture for the details; and secondly there must be a proper 

 screen, i. e., the screen must be very white and very smooth, but 

 not shiny ( 409, 621). White cardboard answers well. Finally 

 there must be an ocular used, and the observers must be near 

 enough the screen to see the fine points. 



FIG. i5oA. ACHROMATIC, SUBSTAGE CONDENSER WITH 



CENTERING SCREWS. 

 (From Zeiss' Catalogue). 



There has been a segment of the condenser cut away to show the construc- 

 tion. 



The centering screws (c-s, c-x) enable the operator to get the condenser in 

 the optic axis of the microscope. The iris diaphragm for this condenser is 

 between the lower and middle combinations, not below the condenser as with 

 the Abbe form. 



This form of condenser is especially desirable for projection and for photo- 

 micrography. 



The substage condenser for micro-projection must either be of a 

 special form to use with the main condenser of the apparatus or 

 special means must be employed to utilize the light cone from the 

 main condenser when the ordinary substage condenser is used. 



This is because the substage condenser ordinarily used on micro- 

 scopes is designed for approximately parallel beams of light, not 

 for those markedly converging or diverging. By examining the 

 figures of the light cone from the main condenser it will be seen 



