240 OX THE ELEMENTARY THEORY OF OPTICAL INSTRUMENTS. 



In one important class of instruments there are no. principal foci or focal 

 centres. A telescope in which parallel rays emerge parallel is an instance. In 

 such instruments, if m be the angular magnifying power, the linear dimensions 



of the image are - of the object, and the distance of the image of the object 



from the image of the object-glass is s of the distance of the object from 



the object-glass. Rules were then laid down for the composition of instruments, 

 and suggestions for the adaptation of this method to second approximations, and 

 the method itself was considered with reference to the labours of Cotes, Smith, 

 Euler, Lagrange, and Gauss on the same subject. 



