206 HERSCHEL. 



principal mirror. When tlie small mirror on the surface of which the 

 second reflection is effected is plane and inclined at an angle of 45^ to 

 the axis of the telescope ; when the image is reflected laterally through 

 an opening made near the edge of the tube and furnished with an eye- 

 piece ; when, in a word, the astronomer looks definitively in a direction 

 perpendicular to the line described by the luminous rays coming from 

 the object and falling on the center of the great mirror, then the tele- 

 scope is called Neiotonian. But in the Gregorian telescope the image 

 formed by the principal mirror falls on a second mirror, which is very 

 small, slightly curved, and parallel to the first. The small mirror re- 

 flects the first image and throws it beyond the large mirror, through an 

 opening made in the middle of that mirror. 



Both in the one and in the other of these two telescopes, 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 con- 

 tributing toward 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 number of rays that the 

 aperture of the telescope had received may be considered as reduced to 

 half. Nor is the diminution less on the small mirror. Now, half of a half 

 is a quarter. Therefore the instrument will send to the eye of the ob- 

 server only a quarter of the incident light that its aperture had received 

 If these two causes of diminished light did not exist in a refracting tele- 

 scope, 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 by 

 jflacing the large mirror obliquely in the tube which causes the images 

 to be formed, not in the axis of the tube, but very near the circumfer- 

 ence or edge of the outer mouth, as we may call it. The observer might 

 therefore look at them directlj' merely by means of an eye-glass. A 

 small portion of the astronomer's head, it is true, encroaches on the tube, 

 forms a screen, and interrupts some incddent rays. Still, in a large tel- 

 escope, this loss does not amount to half as much as it would inevitably 

 do if the small mirror were there. 



Those telescopes in which the observer, standing at the anterior ex- 

 tremity of the tube, looks directly into it, turning his back to the objects, 

 were called by Herschel /row^^Jietc telescopes. In Volume LXXVI of the 

 Philosophical Transactions, he says that the idea of this construction 

 occurred to him in 177G, and that he then applied it unsuccessfully to a 

 ten-foot telescope ; that during the year 1784 he again made a fruitless 

 trial of it in a twenty -foot telescope. Yet I find that on the 7th of Sep- 

 tember, 1784, he recurred to a front view in observing some uebulse and 

 groups of stars. However discordant these dates may be, we cannot 



*It Tvould bo more correct to say lour times as much light. — Translator. 



