433 LECTURJR XXXVI. 



proportion as the dispersive powers of the respective substances, when the 

 mean deviations of the pencils are equal; that is, in the case of the kinds 

 of glass commonly used, nearly in the ratio of 7 to 10. Sometimes also 

 the chromatic aberration, that is, the error arising from the different re- 

 frangibilities of the different rays, is partially corrected in an eye piece, by 

 placing a field glass in such a manner, as considerably to contract the di- 

 mensions of the image formed by the least refrangible rays, which is nearest 

 to the eye glass, and to cause it to subtend an equal angle with the image 

 formed by the most refrangible rays, this image being little afliected by the 

 glass. (Plate XXVIII. Fig. 414, 415.) 



The apparent magnitude of an object, viewed through a telescope, may 

 be measured, with great accuracy, by a scale or by wires, introduced at the 

 place of the last image, reducing afterwards the angle thus ascertained 

 according to the magnifying power. Care must, however, be taken to 

 avoid as much as possible tbe distortion which usually accompanies any 

 curvature of the image; and the wires, one of which is sometimes made 

 moveable by means of a micrometer screw, must be sufficiently illuminated 

 to be distinctly visible. Sometimes a scale is introduced, which, from the 

 apparent magnitude of a known object, such as that of a man of ordinary 

 height, or of a portion of a wall built with bricks of the usual size, enables 

 ns at once to read ofi^' its actual distance, which is expressed on the scale in 

 hundreds of yards. The angular magnitude of an object, seen through a 

 telescope, may also be found, by viewing at the same time, with the other eye, 

 cither a scale, or any other object of known dimensions, placed at a given dis- 

 tance: the lucid disc micrometer of Dr. Herschel is employed in this man- 

 ner for judging of the magnitude of the celestial bodies. The divided 

 object glass micrometer affords another mode of measurement: the object 

 glass being divided into two semicircular portions, one of which slides on 

 the other; each portion acts as a separate lens, and two images of every 

 part of the object being formed, the angular distance of any two points 

 is determined by bringing their images together, and measuring the dis- 

 placement of the moveable portion of the object glass, which is required 

 for procuring the coincidence. Sometimes also a similar purpose is answered 

 by inserting a divided glass in the eye piece, which acts nearly on the same 



