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. 

 Tramlator. 



