354 



PROCEEDINGS OF THE ACADEMY OF 



[1893. 



ness of the glass plates which is 

 known, we may call it T and 

 say therefore that CI= T . 



cosB 

 Further the angle clf=h If 

 — h i c, but as h I f = u c a or A, 

 their sides being parallel, and 

 h i c=I c K or B they being 

 alternate angles it follows that 

 sin clf=sin (A-B). Substitut- 

 ing now these values of c I and 

 sin c I f in the equation c f=c I 

 sin c I f we obtain c f or a b=T 

 sin (A-B). Inasmuch, however, 



cos B 

 as there are two plates in the 

 ophthalmometer and the size of 

 the image is equal to the dis- 

 placement to the left, plus that 

 Fig. 10. to the right, the size of the image 



or I will be equal to 2 a b=2 T sin (A-B) 



cos B 

 As it is essential in the working of the ophthalmometer by means 

 of the formula just developed, that the thickness of its two glass 

 plates as well at their index of refraction should be determined as 

 accurately as possible, the methods by which these contrasts were 

 obtained will be briefly described. 



In determining the thickness of the glass plates a Pfister spharo- 

 meter was made use of, the general construction of which is well 

 shown in Fig. 10. The object to be measured in this instance, the 

 glass plate, was placed upon a little table at the top of the screw 

 (not represented in fig.) and the latter elevated until the glass plate 

 came in contact with the knife edge a. The level being then in 

 adjustment, the extent of the separation of the little table and the 

 upper knife edge or the thickness of the glass plates was then 

 obtained by observing the distance through which the screw had 

 descended, as given by the divisions on the scales and on the grad- 

 uated circle, each division of the scale corresponding to $ millimeter 

 each division of the circle to iU millimeter. The following resume 

 of 25 determinations of the thickness of each glass plate proves the 

 accuracy of the method made use of: — 



