THE INTERFERENCE SPECTRUM BY REFLEXION. 55 



p. 43), it appears that Professor MASOTTI has determined that in these beautiful spectra 

 the most luminous point is in the centre of the yellow ray. which is itself placed at an 

 equal distance from each extremity of the spectrum. From this central point the lu- 

 minous intensity declines regularly and equally to each end, so that the red limit and 

 the violet limit have both the same luminous intensity, and are the least luminous parts 

 of the spectrum. M. MASOTTI has also shown that the colours of these two limits are 

 formed by ethereal waves, the lengths of which are respectively to each other in the 

 remarkable proportion of 2 : 1. On the frontispiece the interference and prismatic 

 spectra are compared side by side. 



194. Before directing farther attention to the peculiarities of the interference spec- 

 trum. I shall proceed to explain the method of obtaining it which I have found most 

 suitable for chemical purposes. Through a narrow fissure, A, Jig. 4, frontispiece, I 

 direct a beam of light horizontally by a heliostat, and at a distance of twelve feet I 

 receive it upon the surface of a piece of ruled glass, the lines of \vhich are parallel to 

 each other, and rigorously parallel to the heliostat fissure, A. More than a year ago I 

 found that there are practically great advantages in using a reflecting grating over one 

 in which the light is directly transmitted. The glass, B C, is therefore to be silvered 

 with a piece of tin foil, so as to act like a mirror, and it will be found that the tin 

 amalgam upon it copies perfectly all the ruled lines. In this mode of operating there 

 is. therefore, no difficult}' in placing B C so that the ray coming from A falls perpen- 

 dicularly on it, for all that is required to ensure the proper adjustment is to move B C 

 until it is in such a position that the ray, after reflexion by it, returns back to the heli- 

 ostat fissure, A. At a distance from B C of six inches, I place an achromatic object 

 glass. D, in such a position that it shall receive perpendicularly the reflected rays of the 

 spectrum of the first order ; in my arrangement this happens to be the left-hand spec- 

 trum, as the observer looks towards the heliostat fissure, A. The achromatic lens is 

 thus brought as near to the grating as possible, without its edge intercepting the ray 

 coming from A. In the focus of this lens, at E F, a ground glass is placed ; it is adjust- 

 ed in a box, E G, and so arranged that the ground glass may be removed, and any 

 photogenic surface put in its place. This portion of my apparatus is, however, nothing 

 more than the sliding part of a common Daguerreotype camera, which contains the ne- 

 cessarv ground glass and cases for photogenic preparations. 



195. The apparatus being properly adjusted, so that the ground glass receives the 

 rays perpendicularly, the spectrum will be seen depicted upon it in beautiful perfection, 

 exhibiting all its striking peculiarities and its fixed lines quite visible. Every precau- 

 tion should be used to shut out all extraneous light 



196. The descriptions given of the interference spectrum by optical writers refer 

 to the transmitted spectrum ; but these which are obtained by reflexion are, as FRAUN- 

 HOFER showed, like those in all respects. For several reasons, it would be preferable 

 to use parallel lines ruled on a piece of polished steel or speculum metal. Silvered 

 glass, however, ha> the advantage of not changing readily under the influence of the 

 various corrosive vapours used in these researches. The grating which I employ is five 

 eighths of an inch Ions, and one third of an inch broad. 



