114 



W. K. BRIDGMAN ON THE 



that by cutting off the opposite portion of the cone of rays, and 

 taking a parallel strip transversely between the centre and the cir- 

 cumference, a still better result was obtained ; and this being applied 

 to the hemispherical condenser, it was found to be equally as effective 

 in refraction as by reflection, and the result of experience has since 

 been to indicate the form and dimensions shown in Fig. 2 as afford- 

 ing the most satisfactory effects. 



Fig. 2. 



This diaphragm should be placed directly beneath the lens as in 

 Fig. 3* — the curved line forming the upper edge of the slit is neces- 



Fig. 3. 



sary to compensate for the unequal surface or curvature of the lens 

 above it. It should be so proportioned that when seen from above 

 in the direction of the ray, the two sides of the light should be as 

 nearly parallel as possible. The slit represents in the original a 

 position of about one-sixteenth of an inch above the centre and a 

 width of between two and three-sixteenths at its widest part. It 

 should be arranged transversely, or parallel with the direction of the 



* Fig. 3. Vertical section of Condenser— A the diaphragm or stop, B the 

 hemispherical condensing lens, C two thicknesses of tinted glass placed 

 vertically to the cone of rays, D. 



