1874.] Improvement of the Spectroscope* 309 



kind of inversion, whether by reflections or otherwise, we can neutralize 

 the curvature while doubling the dispersion. 



If we examine the spectrum, as produced by a series of prisms placed 

 in the position of minimum deviation, we necessarily find that the lines 

 of higher refrangibility, also their centres of curvature, lie towards the 

 centre of the polygon which the prisms themselves affect ; and if we arrest 

 the rays at any part of the circuit, and reflect them directly back by a 

 plane mirror, this reflection reverses (right for left) not only the direc- 

 tion of the centre of curvature of the lines, but also the direction of the 

 spectrum itself, both which are consequently doubled in amount after 

 the rays have performed the second, or return, passage through the 

 prisms ; or (conversely) if, after the first passage through the prisms, we 

 reflect the rays so as to pass through a similar set in such manner as to 

 neutralize the curvature of the first set, we shall find the resulting dis- 

 persion reduced to zero. 



The writer of the article having alluded to a difference between the 

 reflection as given by a plane mirror and a prism of (double) total reflec- 

 tion, it may be observed that, so far as the dispersion and curvature are 

 concerned, the cases are practically identical, the difference being that, 

 in the double reflection, there is a vertical inversion of the spectrum, 

 which, however, produces no discernible effect in either the spectrum or 

 curvature of the lines ; and as the spectroscope constructed with the 

 double reflecting prism is known to produce, with double dispersion, 

 double curvature, we here have an additional proof, if such were required, 

 that the single reflecting mirror does the same. 



The remedy, or means of producing straight spectral lines, which I 

 have alluded to, is simply that of constructing the " slit " with curved 

 instead of rectilinear edges. There is but little practical difficulty 

 incurred in construction, and no apparent objection to its use. It may 

 be objected that for each variation of prism-power in use there should be 

 a special slit. It is, however, only in spectroscopes arranged for high 

 dispersion that the curvature becomes objectionable; in such there is 

 seldom a change required, and a single slit of medium balancing-power 

 would probably remove all practical difficulty, or objectionable curvature 

 of the lines. I have found by trial that, when two compound prisms 

 were in use, giving a dispersion from A to H of nearly 14, the spectral 

 lines were straight in a field of one degree, when the radius of curvature 

 of the slit was made T25 inch. 



[Note on the above Paper. 



If a ray of light be refracted in any manner through any number of 

 prisms arranged as in a spectroscope, undergoing, it may be, any number 

 of intermediate reflections at surfaces parallel to the common direction 

 of the edges of the prisms or, more generally, if a ray be thus refracted 



YOL. XXII. 2 B 



