308 Mr. T. Grubb on the [Apr. 30, 



III. " On the Improvement of the Spectroscope." By THOMAS 

 GRUBB, F.R.S. Received April 30, 1874. 



The importance, as an instrument of research, which the spectroscope 

 has reached within a few years, renders any improvement therein 

 a matter of general scientific interest. Hitherto it has been under a 

 disadvantage, which, though slight in amount in those cases in which the 

 dispersive power of the instrument is moderate, becomes a rather serious 

 annoyance to the observer when a number of prisms are used in serial 

 combination, and the curvature of the spectral lines is proportionally 

 increased, and only to be restrained in appearance by using a narrow 

 breadth of the spectrum. 



I have lately thought of a very simple and practical remedy (which 

 may indeed have occurred to others, but which I have not seen men- 

 tioned), whereby those lines are rendered palpably straight in a very 

 large field ; but previous to describing it, it is desirable to refer to a state- 

 ment appearing in the ' Astronomical Notices ' for last month (March), viz. 

 that the spectral lines can be rendered perfectly straight simply by 

 returning them (after their first passage through a series of prisms 

 arranged for minimum deviation) by a direct reflection from a plane 

 mirror ; and, further, that this has been accomplished in a spectroscope 

 in construction for the Royal Observatory. 



Such a statement has, as might be expected, produced several inquiries ; 

 in one case the querist is much interested, viz. by having a very large 

 spectroscope in hand which, from its construction, involves the ques- 

 tion of straight or curved lines resulting. It therefore seems desirable 

 to remove any illusion which may be entertained, by a short considera- 

 tion of the economy of the spectroscope, so far as the question of curva- 

 ture is concerned. 



The curvature of the spectral lines may be considered a function of 

 the dispersion of a prism ; it (the curvature) not only always accom- 

 panies the dispersion, but, further, its character is always the same with 

 respect to the dispersion that is to say, the centre of curvature will be 

 found invariably to lie in the same direction with respect to the direction 

 of the dispersion, the lines being invariably concave towards that end 

 of the spectrum having the more refrangible rays*. This (which admits 

 of the clearest proof) is adequate to show the impossibility that, by any 



* Professor Stokes has indeed investigated a form of compound prism in which the 

 resulting lines are straight, and on the same principle we may combine prisms (using 

 of course media of different optical powers) in which, with a balance of dispersion 

 remaining, the curvature might be found reversed ; but this does not affect the general 

 law. The curvature in that compound prism (which was the result of various trials, and 

 first used in the spectroscope of the Great Melbourne Telescope, and now, I apprehend, 

 in pretty general estimation and use) probably has a less proportional curvature of the 

 lines than the simple prism. 



