Page:Optics.djvu/154

 The field of view is here found by joining the opposite extremities of the glasses by the lines $$Mn, \ Nm,$$ (Fig. 187.) which mark on the first image the extreme points $$p, \ r,$$ to which a ray belonging can fall on the eye glass. The angle $$ thus measures the field of view which is much larger than in the astronomical telescope. The lines $$Mm, \ Nm$$ define the bright part of the field.

171.The construction of this instrument is better seen in Fig. 189, where $$A$$ represents a concave metallic speculum giving an image of a distant object at its focus $$q,$$ where it is viewed through an eye glass $$B.$$

In practice, the image is thrown to one side as in Fig. 190, as otherwise the head of the observer would intercept the best part of the incident light.

The magnifying powder is plainly $$\frac{f}{F'}, \ f$$ being the focal length of the speculum, and $$F'$$ that of the eye glass. Dr. Herschel has constructed telescopes of this kind that magnify several thousand times, but he generally used powers of only 500 or 600 which gave more brightness to the image.

The visible part of the image is bounded by the lines $$Mm, \ Nn,$$ determining the points $$r, \ p,$$ from which, single rays are sent to the eye glass. The field of view is measured by the angle $$rEp,$$ which is nearly the apparent magnitude of the eye glass seen from the speculum, and is of course very small, for which reason there is often attached to telescopes of this kind a small refracting one, of low magnifying power but considerable field, which has its axis parallel to that of the other, and is called a finder, as it serves to direct the large telescope to any desired point, as a particular star.

172.Reflecting telescopes in general have these advantages over refractors:

They are free from chromatic aberration, being subject only to that of the eye glass, which is never considered. Errata