98 Scientific Proceedings, Eoyal Dublin Society. 



optical parts of different construction would render unnecessary 

 any great alteration in the apparatus. 



Principles of Construction. — I will first describe the principle 

 that guided me in the construction of these cameras, and after- 

 wards describe their parts in detail. In working with the ultra- 

 violet spectrum we have to take into account an effect due to 

 the linear dispersion caused by the lenses. This is comparatively 

 inappreciable, or at any rate so small, that in photographing the 

 visible spectrum it can readily be corrected by the ordinary 

 " side swing " of the back of a well-made photographic camera. 

 To the original instrument used by Miller there was no " swing 

 back," so that the focussing screen and photographic plate were 

 placed at right angles to the ray which passed through the centre 

 of lens. Hence, only this one ray could be properly focussed in 

 any position which could be given to the photographic plate. 

 It was found by experiment with a lens of thirty-six inches focal 

 length for the line D, that the difference in focal length between 

 this and one of the most refrangible rays of cadmium, namely — 

 line 25 was about six inches. The focussing screen, therefore, 

 was placed at such an angle that a mean position was secured for 

 all rays lying between the two extremities of the cadmium 

 spectrum. Having regard to what Professor Stokes has said on 

 the subject, it was scarcely to be expected that the lines would 

 all be perfectly in focus, 



" On account of the increasing refraction by the lens of rays of 

 increasing refrangibility, the locus of the foci of the different rays 

 formed an arc of a curve, or nearly a straight line, lying very obliquely 

 to the axles of the pencils coming through the lens." — (Phil. Trans., 

 1863, p. 605.) 



Experiment, however, has proved that the locus of the foci lie 

 so nearly in a straight line of that part of the prismatic spectrum 

 which acts on a photographic plate, that the slightest curvature 

 caused by the pressure of the spring which holds the plate in 

 position in the dark slide is sufficient to throw the lines consider- 

 ably out of focus. 



In January, 1878, a paper by M. Sarasin v/as published 

 (" Archives des Sciences Physique et Naturelles, Geneva," vol. Ixi,, 

 p. 109), giving the result of his determination of the refraction 



