SOME EXPERIMENTS ON THE ZONAL ABERRATION OE LENSES. 751 



into a balloon-shaped patch of light, witli definite outline, illuminated 

 much more brightly at its lowest point which was infact the part nearest 

 to the principal focus. Eig. 2 depicts the gênerai arrangement. 



t. 



Fig. 2. 



The form of the coma is depicted in élévation to the right. Onreversing 

 the lens so that the convex face was presented (obliqnely) toward the 

 lantern, the coma presented the appearance depicted in Kg 3. Fig. 2 is 



Fig. 3. 



known as an outwardly directed coma; Fig. 3 as an inwardly-directed 

 coma; the spreading body of the coma being in the former case distorted 

 from, and in the latter case toward the axis of the lens. The rays as 

 they cross build caustic curves, in the former case at the lower side, in 

 the latter case at the upper side, of the émergent pencil. 



To ascertain what portions of the lens were responsiblefordeflectin gthe 

 light into the various régions of the coma the flat face of the lens was 



