1877.] 



Magnifying -power of the Half -prism. 



39 



inches), its power is extraordinary. "With the first half-prism alone the D 

 lines are distinctly separated, whilst with the two half-prisms their angular 

 distance is (by actual measurement) |° ; and this separation, be it re- 

 membered, is obtained solely by the prisms, without any magnifying-power 

 from an eyepiece. This separation is about equal to that in the large 

 Greenwich spectroscope with its ten compound prisms of 1*6 inch sec- 

 tion and the ordinary eyepiece, whilst the definition is fully as sharp and 

 the brightness far superior. In fact where it is not necessary to magnify 

 the breadth of the spectrum, the miniature spectroscope would appear 

 to be quite equal to the ordinary form, as there is no use in increasing 

 the height of the prisms beyond what is required to fill the pupil of the 

 eye with light. Thus in the miniature spectroscope the prisms need be 

 only 0*2 inch high ; whilst if a magnifying-power of 10 is to be used (in 

 the direction of the breadth of the spectrum) their height should be 2 

 inches to give the same brightness. Except in the case of stars and other 

 objects of very limited apparent size, breadth may be given to the spec- 

 trum just as well by increasing the height of the slit as by increasing the 

 height of the prisms. Of course the breadth of the first half -prism should 

 be as large as may be, the magnifying-power in this direction being in all 

 cases high enough to reduce the breadth of the pencil to a small fraction 

 of the diameter of the pupil of the eye. 



The results obtained when the train of prisms is reversed so as to give 

 great purity are even more remarkable. With either of the half -prism 

 trains in this position, without slit, collimator, or telescope, the sodium 

 lines are seen with great brilliancy (though not divided) in the flame even 

 of an ordinary paraffin or composite candle without any salt on the wick, 

 and the Fraunhofer lines are seen sharply defined when the train of 

 prisms pure and simple is pointed to the sun or moon. When the two 

 trains are combined as in (5), page 29, still without any slit or lenses, the 

 sodium-lines in an ordinary flame are widely separated, forming two 

 images of the flame ; and the d and b lines in the sun are also well divided. 

 From the low altitude of the sun and the prevalence of haze, I have not 

 had a fair opportunity of trying whether the prominences could be seen 

 in this way without any slit or lenses; but from the ease with which the 

 C line is seen, I think it probable that the prominences would be visible 

 as a ring of light on the background of the spectrum formed by the suc- 

 cessive coloured images of the sun's disk, dazzling though it is. Even if 

 this plan should fail with these short trains of prisms, there would be no 

 difficulty in adding one or two more prisms to each ; for I imagine that 

 the limit to the power is far from having been reached in the spectroscopes 

 I have described. I have deferred making this new plan of spectroscope 

 known until I could try it in a tolerably complete form, although the 

 first experiments I made with direct-vision and other half-prisms in 

 October last convinced me of its practical value. The results which have 

 been obtained so far will, I hope, show that there is no difficulty in ap- 



