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 @? 
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 
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 
1 
with the divergence , and hence, if ¢ = its distance from 
: eee oe 
the image oF +=, and the magnifying power, 
Danes sie is 
M=— =—+ 
BE BOS 
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 f = 0, 
the latter term still expresses it. With this adjustment of the 
lenses, the dynameter gives an expression of the power still 
wider from the truth. 
