nutting: reflection ocular 



405 



and sensibility. The same device serves as a most excellent 

 photometric ocular. 



In the ordinary Gauss ocular, an unsilvered glass plate, set 

 at an angle of 45° with the axis, throws light from the side forward 

 along the axis. The cross hairs are illuminated by this beam 

 passing forward while the field is illuminated by light reflected 

 back from the objective. The reflected image of the cross hairs 

 is a shadow on a bright ground, brightly or faintly illuminated 

 according to the aperture of the reflected beam. The setting is a 

 superposition of the hairs on their shadows, both on a moderately 

 illuminated field, conditions unfavorable to high precision par- 

 ticularly if the reflected beam be narrow. The reflected image is 

 further very difficult to pick up under unfavorable conditions. 



A 



Fig. 1. New reflecting ocular 



In the improved form of ocular, figure 1, a thin elongated 

 reflecting prism is. introduced in front of the ocular as shown in the 

 diagram. This reflecting prism is of microscope cover glass 0.15 

 to 0.20 mm. thick, about 5 mm. wide, and 15 mm. long. Such 

 prisms are easily prepared 50 at a time by grinding a pile of 

 cover glasses clamped together. The sleeve into which the 

 ocular slides is provided with a short side tube. Into this fits a 

 metal plug with a knurled head, half the plug being cut away. 

 To the remaining half the reflecting prism is attached with soft 

 wax. 



This ocular gives a brilliant reflected image easily seen in any 

 part of the field of view. Owing to the smallness of the entering 

 beam the field illumination is low. The setting is a superposition 

 of the black rear face of the prism on its bright reflected image. 



