142 Prof. J. C. Draper on the Projection of the 



15. The theory of coresol vents shows that if in (10) we take 



where the modulus of the elliptic integral is cos 15°, then, if 

 a = 0, the equation is soluble*. If we rationalize it by changing 

 x into cos am at, it becomes 



(1 _^ { 1_^1_^ } + P | +Qy=0, 



where 



^{^%- 1+ {jl +l »w 



and 



\/3 \v/3 



and c= cos 15°. This form is not binomial. There is another 

 soluble form when, with the same value of z y we have 



1 n sCdx 

 a J z£' 



the sign of integration including an arbitrary constant. 



" Oakwal " near Brisbane, Queensland, 

 Australia, November 20, 1874. 



XVIII. Projection of the Fraunhofer Lines of Diffraction and 

 Prismatic Spectra on a Screen. By Prof. John C. Draper, 

 College of the City of New Yorkf, 



HAVING been engaged during the past year in making photo- 

 graphs of absorption-spectra of organic bodies, in which a 

 solar spectrum with Fraunhofer lines was formed by a diffraction- 

 grating, I have resorted to the following method of forming such 

 solar spectra, a description of which may prove of interest to 

 those who are experimenting in the same field. 



The grating generally used was made by Mr. L. M. Ruther- 

 furd : it is ruled on speculum-metal, 6481 lines to the inch ; it 

 gives spectra by reflection. Other gratings on glass, now in my 

 possession, give spectra by reflection and by transmission. The 

 method answers equally well for both. It may be briefly stated 

 as follows : — > 



* See the ' Educational Times,' September 1874, p. 137, and the ' Mes- 

 senger of Mathematics ' there referred to. 



t From the American Journal of Science and Arts, vol. ix. 1875. 



