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XIX. On an Improvement in the Construction of the Spectroscope. 

 By H. G. Mad an. 



To the Editors of the Philosophical Magazine and Journal, 

 Gentlemen, 



IN" the 'Proceedings of the Royal Society/ No. 152, p. 308, 

 there is an account by Mr. Grubb of a very satisfactory 

 method of correcting the curvature of the spectrum-lines, a de- 

 fect inherent in all spectroscopes as at present made. This dis- 

 tortion is due, of course, to the fact that the rays from different 

 parts of the slit fall on the prism under different vertical angles ; 

 and Mr. Grubb proposes to correct it by making the slit itself 

 curved instead of straight, the distorting effect of the prism 

 being then simply employed in rendering straight the slit-images 

 which form the spectrum. 



I think it just worth while to mention, in corroboration of 

 Mr. GrubVs paper, that the same sufficiently obvious remedy 

 occurred to me some time ago, and that since November last I 

 have had curved slits in use for a lantern-spectroscope with per- 

 fectly satisfactory results. These are screwed on (in front of) 

 the ordinary slit-plates, which latter are opened wide ; and the 

 curved plates are thus readily replaced by others by loosening a 

 couple of milled-head screws. Any spectroscope may in this 

 way have the additional slit-plates fitted to it with very little 

 difficulty or expense. 



I have two pairs of slit-plates with curved edges thus fitted to 

 my original slit: — one with edges curved to a radius of 21 cen- 

 tims., which sensibly corrects the distortion of a single carbon- 

 disulphide prism, the refracting angle of which is 60° ; the other 

 slit has a radius of curvature of 10 centims., and is used with a 

 train of two similar prisms. In using such slits with a conden- 

 sing-lens between them and the prism, they should be so placed 

 that the centre of curvature is on the side towards which the 

 rays are refracted by the prism. The above curvatures were de- 

 termined empirically by trials with tinfoil slits, which were easily 

 made by attaching a piece of tinfoil to a plate of glass with gum, 

 and (before the gum was dry) cutting out very narrow strips by 

 a knife fixed to one leg of a pair of beam-compasses. In this 

 way a number of trial slits may be made and tested ; and when 

 the curvature of that one which performs best is noted, any 

 good optician will make a pair of brass plates with edges of the 

 proper form. 



I remain, 



Yours faithfully, 



II. G, Madan. 



Eton College, July 18, 18/4. 



