Manchester Memoirs, Vol. Iv. (191 1), No. *X\. 9 



The lens whose power is to be determined is placed as 

 near as possible in contact with the disc. If the lens 

 is convex it should be placed in the holder, which will 

 press it against the disc. If the lens be concave, it is 

 simply laid on the disc. The lens must be adjusted so 

 that on looking through the pin-hole the centres of the 

 aperture and the circles appear to coincide. The number 

 of circles now in the field of view {Fig. 6) depends upon 



Fig. 6. — Scale of Dioptriemeter seen through 

 a convex spherical lens. 



the power of the lens, and the scale reading of the outer 

 circle seen gives the value of the power of the lens at once 

 in dioptries. To facilitate the estimation of the fractions 

 of a dioptric, a portion of the disc at one side of the aper- 

 ture is cut away, so as to expose a portion of the circles 

 which would otherwise be hidden, and a fine wire is 

 stretched across the gap at the place where the readings 

 must be taken ; in Fig. 6 the value is seen to be +7"8 D. 



The theory of the method follows from the formulai 

 already given. Since the radius of the aperture is 10 mm. 

 or 10 scale divisions, and the distance from the aperture 



