Measurement of Astigmatism and other Ocular Defects. 427 



directly proportional to their absolute critical temperatures 

 and inversely proportional to their critical pressures. 



3. When the critical pressures of two dissimilar substances 

 are unequal, the relation between their molecular volumes on 

 the one hand and their critical temperatures on the other is, 

 as a rule, not so simple as in the previous case. 



XL VII. On the Measurement of Astigmatism and some other 

 Ocular Defects. By John Bridge, M. A.* 



1. TN seeking for the best way of determining defects of the 

 -L eye with respect to refraction, especially hypermetro- 

 pia combined with astigmatism, I was led, in 1874, to adopt a 

 method which, though simple, seems practically sufficient for all 

 ordinary cases. I aimed chiefly at satisfying the conditions : — 



(1) That change of divergence in the pencils issuing from 

 points in the object viewed should be obtained without change 

 in the apparent magnitude of the object. 



(2) That equal changes of divergence (or convergence) 

 should be represented by equal displacements on a scale. 



(3) That the different parts of the object presented should 

 without change in their relative positions in the field of view 

 be seen by pencils in gradually different states of divergence, 

 so that different degrees of clearness should be offered simul- 

 taneously and side by side. 



Moreover, the arrangement must be such that when an 

 observation is repeated, any prejudice which the observer 

 might form from one observation should not be allowed to 

 affect the next. 



2. C is the centre of an achromatic convex lens, at one end 

 of a tube having a length equal to the focal length / of the 

 lens. The tube is fixed to a bar A B, which is parallel to the 

 axis C F of the tube. On the bar is a small table D E which 

 slides along the bar and serves to support the object to be 



viewed. The position of the table is determined by an index 

 at its middle, which traverses a scale on the side of the bar. 

 This scale is divided thus : — from C , which is the projection 

 of C, measure a length (70 equal to the focal length C F. 



* Communicated by the Author. 



