336 LECTURE XXXVI. 



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 repre- 

 sented, 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, pro- 

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

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

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

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

 considered as an image of the object glass. But in the Galilean telescope 

 this method cannot be employed, since no such image is formed. The 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 magni- 

 tude 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 

 glass is, therefore, usually placed, both in telescopes, and in the common 

 compound 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 adjusted by experiment, for several circumstances are con- 

 cerned in the performance of a telescope, which are almost too intricate for 

 practical calculation, although some assistance may certainly be obtained 

 from theory with regard to the most important of them. The curvature of 

 the image produced by any lens has already been mentioned : it may be 

 in some measure remedied by Mr. Ramsden's method of placing a plano- 

 convex lens a little beyond the image, with its flat side turned towards it. 

 Mr. Ramsden* also employs an -eye piece constructed on this principle in- 

 stead of a simple microscope, under the name of a double magnifier. The 

 aberration of the different parts of any single pencil of rays, from the cor- 

 responding point of the image, requires also to be considered in the con- 

 struction of telescopes : its magnitude is such, in the case of a double convex 

 lens of crown glass, that those parts of a pencil of parallel rays which fall 

 on it near the circumference meet each other in a point, which is within 

 the true focus, by a distance a little more than half as great again as the 

 thickness of the lens. In an image formed by a concave speculum of equal 

 * Ph. Tr. 1783, Ixxiii. 94. 



