250 



finding that Lord Rosse's six feet showed without an eye-glass 

 the components of Castor more than a diameter and a half of 

 the larger apart. A still more decisive case is his seeing £ 2 

 Caneri wide double, and the third star as an elongation of its 

 neighbour, though so close. The cause is obvious, but as its 

 effects in large telescopes cannot be neglected, he asks per- 

 mission to state the necessary correction. 



In the telescope an image formed by the objective part is 

 viewed at a small distance by the ocular part. It is commonly 

 assumed that this distance is the focus of a lens equivalent to 



F 



the ocular part, and hence the magnifying power, = — , the 



ratio of the focal lengths of the objective and ocular. This ra- 

 tio is easily shown to be that of the diameters of the objective, 

 and its image formed by the ocular ; and, therefore, the com- 

 mon method of determining the power is to measure the dia- 

 meter of this image by a dynameter, and divide by it that of 

 the objective. 



But in examining a minute object, we do not place it in 

 the principal focus of the lens, or see it by parallel rays. With 

 the unaided eye it is always placed at a certain distance V, 

 which, I believe, in most eyes is about six inches ; the ocular 

 must, therefore, be placed so that the rays shall enter the eye 



with the divergence -zp and hence, if <j> = its distance from 



1 1 1 



the image — = -j. +— , and the magmfying power, 



™ F F F 



The theoretical magnifying power must therefore be increased 

 by the ratio of the focal length of the objective to the least 

 distance of distinct vision, and if there be no ocular or/= oo , 

 the latter term still expresses it. With this adjustment of the 

 lenses, tbe dynameter gives an expression of the power still 

 wider from the truth. 



