on the Light of the Cassegrainian Telescope y ^c. 243 
last distance, as will be seen, was varied in the course of the 
experiments. Two small disks of card paper a quarter of an 
inch in diameter, supported on wires, were fixed on one of the 
sliders, one inch and three quarters from each other, and both 
at the same distance from the mirror. The slider being placed 
between the two foci of the lamps, they were arranged so 
that the reflected light from the most distant lamp fell on one 
of the disks after the rays had crossed at the focus, and that 
from the nearest lamp was received by the other disk before 
the rays arrived at the focus. The slider was then moved 
between the foci till the disks appeared equally illuminated, 
when the division (the rod being divided into inches and 
tenth.s) cut by the slider was registered. The slider was 
then moved till a well defined image of the furthest lamp ap- 
j)earcd on the disk, and the division cut by the slider being 
noted, the distance of the disks, when equally illuminated, from 
the focus ^of the furthest lamp was readily obtained. The foci 
of the two lamps being calculated, their difference was known, 
and by means of it, and the distance of the disks from the focus 
of the furthest lamp, their distance from the focus of the 
nearest lamp was determined. The mean of several readings 
was in every instance taken to insure a more accurate result, 
and the lamps were changed during each experiment, in 
order to obviate any error which might arise from the inequa- 
lity of their light. The rays from the distant lamp were first 
thrown on the right hand disk, and in the subsequent series 
on the left; by which the error arising from any shade of dif- 
ference in the disks (if such existed) was corrected. 
Let L, be the distance of the furthest lamp. 
1 , TJiat of the nearest. 
I i 2 
