Focometry of a Concave Lens. 
109 
adjusted until the rays reflected by the plane mirror M, passing- 
through the lenses again, form an image of the cross-wires on 
the screen carrying them. 
Fig. 1. 
F is then at the principal focus of L and the distance 
between L and F is the focal length. 
II. As in the first method, light from the cross- wires G 
(fig. 2) is brought to a focus at F. The concave lens L is 
Fig. 2. 
■----,---H 
tli en inserted and adjusted until an image of the cross-wires 
is formed in the focal plane of the telescope T, which has 
been previously focussed for parallel rays. The distance 
between L and F is the focal length as before. 
III. In this method a distant object is viewed through the 
concave lens L by means of the telescope T (fig. 3). The 
Fi<?. 3. 
latter will therefore be focussed as for an object at F, the 
principal focus of L. 
