4^0 LECTURE XXXVI. 



considerably less curved than that which is depicted by a lens of equal focal 

 length. There is a similar imperfection in the nature of the focus of oblique 

 pencils, but it is confined within narrower limits, the remotest part of the 

 image in which any radiating lines would be most distinctly represented, 

 being a flat surface, and the nearest, in which circles would become most 

 distinct, being a part of a sphere touching the speculum : so that the radius 

 of the mean curvature is equal to the focal distance. (Plate XXVIII. Fig. 

 411.) 



The magnifying power of a refracting telescope may often be measured, by 

 comparing the diameter of the object glass with that of the narrowest space, 

 into which the beam of light is contracted beyond the eye glass, provided 

 that none of the light has been intercepted in its passage through the tele- 

 scope: for the object will be viewed through the telescope in an angle as 

 much greater tlian that which it naturally subtends, as the diameter of the 

 object glass is greater than that of this contracted pencil, which may be con- 

 sidered as an image of the object glass. But in the Galilean telescope, this 

 method cannot be employed, since no such image is formed. Th? field of 

 view, in a simple telescope, or the angular magnitude of that part of an 

 object which can be seen through it at once, is nearly equal to the magnitude 

 of the eye glass as seen from the object glass. 



If a lens be added to any refracting telescope at the place of the first 

 image, it will have no effect either on the place or on the magnitude of any 

 subsequent image, but it will enlarge the field of view, by throwing more 

 pencils of light on the original eye glass. If, however, the image fell 

 exactly on such a lens, it would be liable to be impaired by any accidental 

 impurities of its substance or on its surface, every opaque particle inter- 

 cepting the whole of the light belonging to one of its points, which would not 

 happen if the image were at a small distance from the lens. A field gLs« 

 is, therefore, usually placed, both in telescopes, and in the common com- 

 pound microscope, a little nearer to the object glass than the place of the 

 first image. The best places for the various lenses, in an eye piece, are partly 

 determined from similar considerations, but they require also in general to be ad- 

 justed by experiment, for several circumstances are concerned in the perform- 



