PLATE 11. 



Low-power Photomicrography. 



EMBRYO CHICK, 54 hours' incubation x 15 



Objective 2" by Grouch. Stopped down. 



Ocular None. 



Condenser ... Back combination of Watson's Universal 



Achromatic. 



Illuminant ... Incandescent gas (inverted mantle). 



Screen Green glass. 



Plate Edwards' Medium Iso. 



Camera Ext. ... 32". 



Exposure l^min. 



An example of low-power photomicrography with an achromatic 

 objective, illuminated by means of the back combination of an 

 achromatic condenser. 



In order to obtain depth of focus the objective was stopped down 

 by placing a circular metal diaphragm having a small central 

 aperture between the two lenses of the objective combination. 



The preparation was stained with carmalum, and, as many of 

 the finer blood vessels in the clear area (area pellucida) were only 

 faintly stained, a green pot glass screen was used to increase the 

 contrast of the paler structures. This had the effect, however, of 

 obscuring some of the detail in the more densely stained portions. 



Photo W. B. R. \Tofollowpage 112. 



