in the Solar Spectrum. 291 



the preference is to be given to one of flint glass with a large 

 refracting angle (say 50 or 60), because of its high refractive 

 and dispersive powers ; all that is required is, that it should 

 approach as near to perfect homogeneity as possible. A few 

 streaks running parallel to the refracting edges may be disre- 

 garded ; but if they are thrown into waves or curved lines, by 

 injudicious workmanship, the prism is utterly useless for this 

 purpose. Such a prism being obtained, let it be placed twenty, 

 thirty, or more feet distant from, and with its refracting angle 

 parallel to a very narrow linear opening in the window shutter 

 of a darkened room : such an aperture or opening may be con- 

 veniently formed in a piece of tin foil, or thin sheet lead, by the 

 point of a sharp penknife ; and means must be taken to illumi- 

 nate this aperture either by the rays of the sun reflected from 

 the surface of a plane mirror, or by the light from a bright sky : 

 the former is decidedly to be preferred ; that part of the solar 

 miscroscope which is employed for a similar purpose when 

 that instrument is used, will be found very convenient for this. 

 The prism being placed in the beam of light transmitted by the 

 aperture, either in a vertical or horizontal position, and its re- 

 fracting angle as nearly parallel as possible to the aperture, turn 

 it round on its axis until the refracted spectrum of the illumi- 

 nated aperture formed by the prism appears to be stationary ; 

 the prism is then in the position of minimum deviation, and 

 the most favourable one for the production of the lines. 



If the spectrum be now carefully examined, a number of 

 narrow black stripes will be seen crossing it at various distances 

 from each other ; but as there is some difficulty in seeing them 

 with the unassisted eye, on account of their minuteness, it is 

 better to employ a telescope for the purpose, which, however, 

 need not be of large dimensions, nor possess very high mag- 

 nifying power ; the one I generally employ for this and similar 

 purposes is J/6 inch aperture, and 18 3 inches focal length, 

 mounted on the common portable stand, and has amplifying 



broad, with which I have seen the lines very distinctly ; also in a small triangular 

 prism of rock crystal, with its refracting angles parallel to the axis of the original 

 prism, I have seen the lines both in the ordinary and extraordinary spectrum of 

 the aperture perfectly sharp and well-defined. 



