404 M. H. Helmholtz on Sir David Brewster's 



to show the lines of Fraimhofer, the coloured rays changed by 

 the absorbing medium^ and to which the experiments had refer- 

 encCj are for the most part so feebly luminous that they can be 

 clearly seen by direct sunlight only. 



The doubts which impressed themselves upon me dming the 

 repetition of these experiments, refer, in the first place, to the 

 question whether small quantities of white dispersed light might 

 not have been mixed up with the spectrum ; and secondly, 

 whether the ej'e under the given circumstances was not prevented 

 by physiological influences from forming a correct judgement as 

 to the colours. "With regard to the doubt flrst expressed, it may 

 be stated that by the method of Brewster all light, with the ex- 

 ception of that which enters through the aperture, may be com- 

 pletely cut off; w^ith a good strongly refracting prism, or a com- 

 bination of two such prisms and a narrow slit, it is possible, so 

 far as the solar light is regularly refracted, to separate it very 

 completely into its differently coloured rays, so that in the spec- 

 trum these shall not at all overlap each other. It must, however, 

 be remembered that a small portion of light could obtain admis- 

 sion by another way than that of regular refraction. In the first 

 place, the dispersion of the light at the limiting surfaces and in 

 the mass of the glass merits consideration. 



If a piece of glass, whether a prism or a lens, be directly shone 

 upon by the sun and observed against a dark ground, no matter 

 how clear or how highly polished it may be, a great number of 

 shining points are always seen in its interior, and minute scratches 

 upon its surface ; both disperse irregularly a sensible quantity 

 of light, and impart to the whole a smoky appearance. To 

 render such an examination quite exact, let a sheaf of solar rays 

 issuing through an orifice in a dark screen fall iqjon the piece of 

 glass, and let the eye be brought nearly into the same line as the 

 transmitted rays, so that the latter shall pass close to the eye 

 but not enter it. The little irregularities of the surface and 

 mass then appear brightly illuminated against the black ground 

 furnished by the screen. The flint-glass prism cut by Plossl, 

 which I made use of, and which with a telescope showed the lines 

 of Fraunhofer in great number and perfection*, was not free 

 from such irregularities. Brewster himself has not left this 

 point unnoticed ; in his reply to Draper he observes, that besides 

 the most beautiful prisms of glass, he also used prisms of rock- 

 salt, of such homogeneity and purity that on looking through 

 them the substance of the prism was invisible ; but he does not 

 say whether he tested them by direct sunlight in the manner I 

 have described. In this way many imperfections are rendered 



* It resolved, for example, the line D into its two component lines lying 

 close beside each other. 



