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slight distance from the axis of the telescope. Hence in the 
two first cases the object is viewed in the usual 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 ; and in the last the 
observer sees the object inverted, but not altered as respects 
right and left, by the front view, so called, I suppose (quasi 
lucus a non lucendo), because the observer's back is turned 
towards the object. 
It appears, then, that in all astronomical telescopes, reflect- 
ing 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 screen 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 single convex lens of long focus 
is the simplest telescope.* But a singular circumstance will 
immediately attract the observer's notice. A real picture, or 
an image formed on a screen, as in the former case, can be 
viewed at varying distances, but when the image is viewed 
directly it will be found that for distinct vision the eye must be 
placed exactly, or almost exactly, at a fixed distance from the 
image. This is more important than 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 how an image formed by 
an astronomical object-glass, or mirror, differs (as respects 
visibility) from a real object. The peculiarities to be noted 
are the curvature, indistinctness, and false colouring of the 
image. * 
not,” says Sir Isaac Newton, “that Mr. Gregory could have described morn 
fashions than one of these telescopes, and perhaps have run through all the 
possible cases of them, if he had thought it worth his pains.” 
* Such a telescope is most powerful with the shortest sight. It may be 
noticed that the use of a telescope often reveals a difference in the sight of 
the two eyes. In my own case, for instance, I have found that the left eye 
is short-sighted, the sight of the right eye being of about the average range. 
Accordingly with my left eye a 5^-foot object-glass (alone) forms an effective 
telescope, with which I can see Saturn’s rings and Jupiter’s moons quite dis- 
tinctly. I find that the moon is too bright to be observed in this way with- 
out pain, except at low altitudes. 
* 
