144 Royal Society : — 



extremities of each spectrum. In the spectra of the first order 

 on each side, the screen is placed very nearly at right angles to 

 a line drawn from the grating to B in the spectrum. As each 

 order in succession is examined, the divergence from this angle 

 is greater and greater, and at the same time the focal distance 

 of the lines moves nearer to the grating. 



The lenses I have employed were those of a very fine photo- 

 graphic combination ; they give with the rest of the arrangement 

 a spectrum in which the definition of the lines is perfect, and 

 they are present by hundreds. Though the lenses are 10 centims. 

 in diameter, only the central portion of each is used, a diaphragm 

 with a circular aperture of 5 centims. or less being placed in 

 front of B. 



To form the absorbent spectra of any organic substance, a 

 suitable solution of the same is poured into a cell with parallel 

 sides. This is placed at any convenient point between A and B, 

 care being taken that the faces of the cell are at right angles to 

 the course of the ray A, B. The slit may in this case be opened 

 wider, when each spectrum will show the characteristic absorbent 

 bands of the substance employed, the position being indicated 

 (and, if required, recorded) by their relation to the lines of the 

 solar spectrum in which they are produced. 



When the calcium or electric light is to be used for lecture- 

 room demonstration of diffraction-spectra, the lens (B) should 

 have as short a focus and as large a diameter as possible. The 

 grating may also be so arranged on its vertical axis as to throw 

 its image at a right angle to the line B E, to be there received 

 on a screen. Though by this device the spectra on one side of 

 the image of the grating are greatly elongated, and those on the 

 other compressed, it presents the advantage of enabling the 

 audience to see all the spectra at once, and also the optical con- 

 trivances by which they are produced. 



XIX. Proceedings of Learned Societies. 



ROYAL SOCIETY. 

 [Continued from vol. xlviii. p. 541.] 



May 7, 1874. — William Spotfciswoode, M.A., Treasurer and Vice- 

 President, in the Chair. 

 THE following communication was read : — 

 Addition to the Paper, " Volcanic Energy : an attempt to 

 develop its true Origin and Cosmical Relations "*. By Robert Mal- 

 let, A.M., C.E., F.R.S., M.R.I.A., Ac. 



Referring to his original paper (Phil. Trans. 1873), the author 



* Phil. Mag. vol. xliv. p. 468. 



