Dr. J. Reade un a new T/icnnj v/' Telescopes. 2f> 



ready remarked, that the straight shadow of a knitting-needle 

 became curved by striking and passing through the prism ; 

 consequently a circle would become oblong : but what at first 

 did surprise* me was, that the entire colours of the spectrum 

 were reversed from what I saw on looking at the hole through 

 the prism. The violet was now at top, the orange at the bot- 

 tom. This fact escaped the notice of Newton and his fol- 

 lowers: had they noticed it, the theory of repulsion and attrac- 

 tion must have undergone some slight modification. At some 

 future time I shall endeavour to account for this phenomenon. 

 And I again repeat, that any person performing this experi- 

 ment must be satisfied that the colours are produced by those 

 fringes, and also that those fringes are produced by the rare- 

 faction of black light, and by no means from any separation 

 of the solar ray. 



"I shall now say a few words on the astronomical telescope. 

 Mr. Woods has the following description, p. 160: — 



" Let L and E be the centres of the two glasses; Q P an 





object towards which the axis of the telescope is directed, and 

 so distant that the rays which flow from any one point in it, 

 and fall upon the object-glass L, may be considered as pa- 

 rallel. Then qp, an inverted image of Q P, will be formed 

 in the principal focus of the glass L, and contained between 

 the lines QLyand PL;>; and because LE is equal to the sum 

 of the focal lengths of the two glasses, p q is in the principal 

 focus of the glass AB; consequently p Q maybe seen di- 

 stinctly through the glass, if the eye of the observer be able 

 to cojlect parallel rays upon the retina. Produce pL till it 

 meet in the eye-glass in B, join p E, and draw BO parallel to 

 p E. Then the rays which flow from p in the object on p its 

 image, enter the eye placed at O in the direction BO; also 

 the rays which flow from Q enter the eye in the direction 

 EO. Thus the angle which QP subtends at the centre of 

 the eye, when viewed through the telescope, is the angle BoE, 

 which is equal to PE q. The angle which QP subtends 

 when viewed with the naked eye from L, is P L Q, which is 

 equal toflLg." The same objections I have already brought 

 against the Galilean telescope arc equally strong here ; I there- 

 fore 



