optics, 309 



compounded light, being differently refrangible, 

 come to their respective foci at different distances 

 from the glass, the more refrangible rays converg- 

 ing sooner than those which are less refrangible. 

 This is confirmed by experiment : for a paper 

 painted entirely red, and properly illuminated, will 

 cast its image by means of a lens, on a screen at a 

 greater distance than will another blue paper by 

 the same lens in like circumstances. And here it 

 may be noted, that the lens proper for this expe- 

 riment must be very flat, or a portion of the sur- 

 face of a large sphere. Hence the image of a 

 white object, maybe said to consist of an indefinite 

 number of coloured images, the violet being nearest 

 and the red farthest from the lens, and the images 

 of intermediate colours at intermediate distances. 

 The aggregate or image itself, must, therefore, 

 be in some degree confused, and this confusion 

 being very much increased by the magnifying 

 power or eye-glass, renders it necessary to use an 

 eye-glass of a certain limited convexity to a given 

 object-glass. For which reason, if it be required 

 to construct a telescope that shall magnify objects 

 in a greater degree than a given telescope, the 

 object-glass must be less convex, and of conse- 

 quence its focal distance longer. 



It is also necessary to limit the aperture of the 

 object-glass to exclude those rays which are inci- 

 dent at too great distances from the centre ; for 

 those, being more refracted, are more particularly 

 subject to the irregularities which arise either from 

 the figure of the glass or the unequal refraction of 

 light. 



The great difficulty of managing the longer tele- 

 scopes occasioned the philosophic world to fix 



x 3 



