274 HERSCHEL. 
Both in the one and in the other of these two tele-— 
scopes, the small mirror interposed between the object 
and the great mirror forms relative to the latter a sort of 
screen which prevents its entire surface from contributing 
towards forming the image. ‘The small mirror, also, in 
regard to intensity, gives some trouble. 
Let us suppose, in order to clear up our ideas, that the 
material of which the two mirrors are made, reflects only 
half of the incident light. In the course of the first 
reflection, the immense quantity of rays that the aperture 
of the telescope had received, may be considered as re- 
duced to half. Nor is the diminution less on the small 
mirror. Now, half of half is a quarter. Therefore the 
instrument will send to the eye of the observer only a 
quarter of the incident light that its aperture had re- 
ceived. These two causes of diminished light not exist- 
ing in a refracting telescope, it would give, under parity 
of dimensions, four times more * light than a Newtonian 
or Gregorian telescope gives. 
Herschel did away with the small mirror in his large 
telescope. The large mirror is not mathematically centred 
in the large tube that contains it, but is placed rather 
obliquely in it. This slight obliquity causes the images 
to be formed not in the axis of the tube, but very near 
its circumference, or outer mouth, we may call it. The 
observer may therefore look at them there direct, merely 
by means of an eye-piece. A small portion of the astron- 
omer’s head, it is true, then encroaches on the tube; it 
forms a screen, and interrupts some incident rays. Still, 
in a large telescope, the loss does not amount to half by 
a great deal; which it would inevitably do if the small 
mirror were there. 
* It would be more correct to say four times as much light.— 
Translator. 
