MODE OF FORMING THE INTERFERENCE SPECTRUM. 53 



the lesser refrangible regions are concentrated. The relative position of the colours 

 must also vary ; the fixed lines must be placed at distances greater than the true dis- 

 tance, as the violet end is approached. 



184. In all investigations on the chemical action of the spectrum, the greatest impor- 

 tance is to be attached to these considerations, for our estimates of chemical results de- 

 pend on the amount of action taking place in a given time. When, therefore, we ob- 

 tain a prismatic impression on any sensitive surface, it is very far from representing the 

 true character of the phenomena. The action which ought to be concentrated in a 

 lesser space at the more refrangible region is spread over a greater, and with that aug- 

 mentation an apparent diminution of the amount of action is perceived. This, of 

 course, should make the maximum point vary, spread out unduly the violet end, and 

 dilute the true effect. The different regions of the prismatic spectrum cannot be fairly 

 compared with one another. 



185. My attention having been directed to these peculiarities, I obtained from the 

 mint at Philadelphia, in May, 1843, a piece of ruled glass, with a view of examining 

 the different questions for which solutions had been apparently obtained by the defec- 

 tive use of the prismatic spectrum, substituting for it the interference spectrum of FRAUN- 

 HOFER. I am particular in mentioning this date, because I perceive that, at the very 

 time that I was making these trials in New- York on the chemical agencies of the spec- 

 trum, Professor MASOTTI was doing the same thing for the luminous rays in Italy, as is 

 stated by M. MELLONI, in the Comptes Rendus, Jan., 1844, p. 44 ; and it is possible 

 that, before the time that this volume reaches the Continent of Europe, other skilful ex- 

 perimenters may have followed in the course of Professor MASOTTI, and, perceiving the 

 great advantages which arise, may have applied it for the chemical rays. 



186. As the interference spectrum is less known than the prismatic, I will here give 

 a brief account of the mode by which it is to be formed for experimental purposes, and 

 of its peculiarities. 



187. The principal apparatus is a grating composed of a number of intervals, alter- 

 nately transparent and opaque. A piece of glass, with lines ruled on it with the point 

 of a diamond, forms the best grating.; these lines should be parallel to one another, equi- 

 distant, and so close that several hundreds, or even thousands of them, may be com- 

 prised in an inch. To so great a degree of perfection did FRAUNHOFER carry this kind 

 of work, that he obtained gratings containing 30,000 lines in the inch. A beam of 

 light, directed horizontally into a dark room by means of a heliostat, is to be trans- 

 mitted through a fissure, as in the arrangement for showing the fixed lines in the pris- 

 matic spectrum. At a distance of several feet, the ray is received on the ruled glass, 

 behind which, a few inches intervening, a piece of ground glass is placed. The lines 

 which are upon the glass are to be adjusted so that they are parallel to the sides of the 

 fissure through which the light enters. In FRAUNHOFER'S arrangement, instead of a 

 ground glass, a small achromatic telescope was placed, having a mechanism for meas- 

 uring angular deviations, and rotating on an axis coincident with the centre of the gra- 

 ting ; the following facts, which may less exactly be verified with a ground glass, were 

 then determined. Upon the axis of the ray coming in through the fissure, a white 



