head, his body, cannot then injure the formation or the brightness of the image, however small may be the distance from which we have to study it.  But it is no longer thus with the image formed by means of reflection.  For the image is now placed between the object and the reflecting mirror; and when the astronomer approaches in order to examine it, he inevitably intercepts, if not the totality, at least a very considerable portion of the luminous rays, which would otherwise have contributed to give it great splendour.  It will now be understood, why in optical instruments where the images of distant objects are formed by the reflection of light, it has been necessary to carry the images, by the aid of a second reflection, out of the tube that contains and sustains the principal mirror.  When the small mirror, on the surface of which the second reflection is effected, is plane, and inclined at an angle of 45 deg. 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 centre of the great mirror, then the telescope is called Newtonian.  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 reflects the first image and throws it beyond the large mirror, through an opening made in the middle of that principal 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 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 reduced 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 received.  These two causes of diminished light not existing in a refracting telescope, it would give, under parity of dimensions, four times more[19] 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 astronomer’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.