Page images

the reason of all the lines which

you see in the figure ?

James. I think I can;there are two pencils of rays flowing from the extremities of the arrow, which is the object to be viewed. The rays of the pencil flowing from X, go on diverging till they reach the convex lens 'o P, when they will be so refracted by passing through the glass, as to converge and meet in the point

Now the same may be said of the pencil of rays which come from y; and, of course, of all the pencils of rays flowing from the object between x and y. So that the image of the arrow would, by the convex lens, be formed at E.

Tutor. And what would happen if there were no other glass? 4. James. The rays would cross each

[ocr errors]
[blocks in formation]

other and be divergent, so that when they got to the retina, there would be no distinct image formed, but every point as x or y, would be spread over a large space, and the image would be confused. To prevent this, the concave lens m n is interposed; the pencil of rays which would, by the convex glass, converge at x, will now be made to diverge, so as not to come to a focus till they arrive at the retina: and the pencil of rays

which would, by the convex glass, have come to a point at y, will, by the interposition of the concave lens, be made to diverge so much as to throw the focus of the rays to b instead of y. By this means, the image of the object is magnified.

Tutor. Can you tell the reason why the tubes, require to be drawn

out more or less for different persons ?

Charles. The tubes are to be adjusted, in order to throw the focus of rays exactly on the retina: and as some eyes are more convex than others, the length of the focus will vary in different persons, and, by sliding the tube up or down, this object is obtained.

Tutor. Refracting telescopes are used chiefly for viewing the terrestrial objects ; two things, therefore, are requisite in them; the first is, that it should show objects in an upright position, that is, in the same position as we see them without glasses, and the second is, that they should afford a large field of view.

James. What do you mean, sir, by a field of view ?

Tutor. All that part of landscape which may be seen at once, without moving the eye or instrument. Now in looking on the figure again, you will perceive that the concave lens throws a number of the rays beyond the pupil .c of the eye, on to the iris on both sides, but those only are visible, or go to form an image, which pass through the pupil; and there fore, by a telescope made in this way, the middle part of the object is only seen, or, in other words, the prospect is by it very much diminished.

Charles. How is that remedied?

Tutor. By substituting a double convex eye-glass g h (Plate v. Fig. 35.) instead of the concave one. Here the focus of the double convex lens is at E, and the glass g h must be so much more convex than o P,

[ocr errors]

as that its focus may be also at E: for then the rays flowing from the object r y, and passing through the object glass o p,, will form the inverted image m E d. Now by interposing the double convex g h, the image is thrown on the retina, and it is seen under the large angle de c, that is, the image m E d will be magnified to the size C E D.

James. Is not the image of the object in the telescope inverted?

Tutor. Yes it is: for you see the image on the retina stands in the same position as the object; but we always see things by having the images inverted : and, therefore, whatever is seen by telescopes constructed as this is, will appear inverted to the spectator, which is a very unpleasant circumstance with

« PreviousContinue »