146 



Mr. J. A. Tomkins on the Direct 



The cross-wires may conveniently be two parallel wires fixed 

 1 cm. apart so that the magnification can be read off directly 

 on a centimetre scale. This method does not apply to con- 

 vergent lenses having a power of less than +1 D, nor to 

 divergent lenses. Neither is it very convenient for con- 

 vergent lenses of less than about +2D on account of the 

 great distance of the cross-wires from the lens. 



The powers of such lenses could, however, be determined 

 indirectly by combining them with a convergent lens of 

 suitable power and then measuring the power of the combi- 

 nation. But the method can be applied directly and con- 

 veniently to divergent lenses by the simple modification of 

 the previous arrangement shown in fig. 2, which has not, so 

 far as the writer is aware, been previously described. 



Fig. 2. 



ferment Lens 



As before, a ray parallel to the principal axis is the locus 

 of successive object-points (not shown) and the divergent ray 

 FI is the locus of corresponding image-points. Hence for a 

 certain position of the object-point the image-point will be 

 at I on a screen 1 metre from the lens. is the point of inter- 

 section of the ray CI through the optical centre with the 

 incident ray. The " object " here is obviously a virtual object 

 formed by rays converging towards and intercepted by 

 the divergent lens. 



In this case we have 



f-f=(m-l)f=l, 



1 



m — 1 = -j, = D, 



whence 



or, the power= (Magnification — 1). The virtual object is 



