Let us compare with the arrangement exhibited in [fig. 1] that adopted by Galileo. Surprise is sometimes expressed that this instrument, which in the hands of the great Florentine astronomer effected so much, should now be known as the non-astronomical Telescope. I think this will be readily understood when we compare the two arrangements.
In the Galilean Telescope a small concave eye-glass, ab ([fig. 2]), is placed between the object-glass and the image. In fact, no image is allowed to be formed in this arrangement, but the convergent pencils are intercepted by the concave eye-glass, and converted into parallel emergent pencils. Now in [fig. 2] the concave eye-glass is so placed as to receive only a part of the convergent pencil A p B, and this is the arrangement usually adopted. By using a concave glass of shorter focus, which would therefore be placed nearer to m p, the whole of the convergent pencil might be received in this as in the former case. But then the axis of the emergent pencil, instead of returning (as we see it in [fig. 1]) towards the axis of the telescope, would depart as much from that axis. Thus there would be no point on the axis at which the eye could be so placed as to receive emergent pencils showing any considerable part of the object. The difference may be compared to that between looking through the small end of a cone-shaped roll of paper and looking through the large end; in the former case the eye sees at once all that is to be seen through the roll (supposed fixed in position), in the latter the eye may be moved about so as to command the same range of view, but at any instant sees over a much smaller range.
To return to the arrangement actually employed, which is illustrated by the common opera-glass. We see that the full illuminating power of the telescope is not brought into play. But this is not the only objection to the Galilean Telescope. It is obvious that if the part C D of the object-glass were covered, the point P would not be visible, whereas, in the astronomical arrangement no other effect is produced on the visibility of an object, by covering part of the object-glass, than a small loss of illumination. In other words, the dimensions of the field of view of a Galilean Telescope depend on the size of the object-glass, whereas in the astronomical Telescope the field of view is independent of the size of the object-glass. The difference may be readily tested. If we direct an opera-glass upon any object, we shall find that any covering placed over a part of the object-glass becomes visible when we look through the instrument, interfering therefore pro tanto with the range of view. A covering similarly placed on any part of the object-glass of an astronomical telescope does not become visible when we look through the instrument. The distinction has a very important bearing on the theory of telescopic vision.
In considering the application of the telescope to practical observation, the circumstance that in the Galilean Telescope no real image is formed, is yet more important. A real image admits of measurement, linear or angular, while to a virtual image (such an image, for instance, as is formed by a common looking-glass) no such process can be applied. In simple observation the only noticeable effect of this difference is that, whereas in the astronomical Telescope a stop or diaphragm can be inserted in the tube so as to cut off what is called the ragged edge of the field of view (which includes all the part not reached by full pencils of light from the object-glass), there is no means of remedying the corresponding defect in the Galilean Telescope. It would be a very annoying defect in a telescope intended for astronomical observation, since in general the edge of the field of view is not perceptible at night. The unpleasant nature of the defect may be seen by looking through an opera-glass, and noticing the gradual fading away of light round the circumference of the field of view.
The properties of reflection as well as of refraction have been enlisted into the service of the astronomical observer. The formation of an image by means of a concave mirror is exhibited in [fig. 3]. As the observer's head would be placed between the object and the mirror, if the image, formed as in [fig. 3], were to be microscopically examined, various devices are employed in the construction of reflecting telescopes to avoid the loss of light which would result—a loss which would be important even with the largest mirrors yet constructed. Thus, in Gregory's Telescope, a small mirror, having its concavity towards the great one, is placed in the axis of the tube and forms an image which is viewed through an aperture in the middle of the great mirror. A similar plan is adopted in Cassegrain's Telescope, a small convex mirror replacing the concave one. In Newton's Telescope a small inclined-plane reflector is used, which sends the pencil of light off at right-angles to the axis of the tube. In Herschel's Telescope the great mirror is inclined so that the image is formed at a slight distance from the axis of the telescope. In the two first cases the object is viewed in the usual or direct way, the image being erect in Gregory's and inverted in Cassegrain's. In the third the observer looks through the side of the telescope, seeing an inverted image of the object. In the last the observer sees the object inverted, but not altered as respects right and left. The last-mentioned method of viewing objects is the only one in which the observer's back is turned towards the object, yet this method is called the front view—apparently quasi lucus a non lucendo.
It appears, then, that in all astronomical Telescopes, reflecting or refracting, a real image of an object is submitted to microscopical examination.
Of this fact the possessor of a telescope may easily assure himself; for if the eye-glass be removed, and a small screen be placed at the focus of the object-glass, there will appear upon the screen a small picture of any object towards which the tube is turned. But the image may be viewed in another way which requires to be noticed. If the eye, placed at a distance of five or six inches from the image, be directed down the tube, the image will be seen as before; in fact, just as a single convex lens of short focus is the simplest microscope, so a simple convex lens of long focus is the simplest telescope.[1] But a singular circumstance will immediately attract the observer's notice. A real picture, or the image formed on the screen as in the former case, can be viewed at varying distances; but when we view the image directly, it will be found that for distinct vision the eye must be placed almost exactly at a fixed distance from the image. This peculiarity is more important than it might be thought at first sight. In fact, it is essential that the observer who would rightly apply the powers of his telescope, or fairly test its performance, should understand in what respect an image formed by an object-glass or object-mirror differs from a real object.