358 



PROCEEDINGS OF THE ACADEMY OP 



[1893. 



and left respectively. The subject of the experiment upon whose eye 

 the light from the three gas jets falls, sits with the head resting in a 

 frame at a distance of about 90 centimeters from the ophthalmome- 

 ter, and looks directly at the center of the 

 frame supporting the three gas jets. Three 

 corneal images being then perceived by 

 the observer looking through the ophthalmo- 

 meter erect virtual, but reversed as in a look- 

 ing glass, the plates of the ophthalmometer 

 are made to diverge until the image 1' 

 reaches a point just midway between the 

 images 2 and 3. The original images, or 

 the distance between 1 and a point midway 

 between 2 and 3, being then doubled, the 

 angle of deviation of the plates, or A, is 

 then read off. The angle A, so obtained 

 experimentally, being substituted in the 

 formula 1=2 T sin (A-B) the size of the 



cos B 

 is then obtained by calculation 



image or I 



as shown by the follovving example : 



1=2 T sin (A-B) 



cos B 

 A=32° 48', T=4-4015 mm. Index of refraction=l-5249 

 sin A 

 =index of refraction=l*5249 



sin B 



sin 32° 48' 



sin B 



1-5249 

 log sin 32° 48=9-733765 

 log 1-5249 =0-183241 



log sin B =9-550524 



B =20° 48' 30" 

 A— B=ll° 59' 30" 



log sin (A— B)=9-317582 

 log cos B =9-970707 



log2T 



log I 



9-346875 

 =0-944631 



=0-291586 

 =1-957 mm. 



