106 Prof. Draper on the Production of Light 



substituted a jet-pipe, through which the various gases might 

 be made lo pass, and the rays emitted by their flames enter the 

 telescope after passing thiough the slit and prism. In this 

 arrangen)ent the slit should be horizontal and not vertical. 

 So far from its being immaterial which of the two positions is 

 selected, very great advantages arise from the former. If the 

 slit be vertical, the prism it is true will separate the constituent 

 colours from one another, but it fails to show their relative 

 position. If it be horizontal, the relative positions of the dif- 

 ferent colours can be demonstrated ; and it can be proved that 

 a horizontal section of a flame is in reality, as has been already 

 remarked, a coloured ring, the red being the innermost colour 

 and the violet outside ; for if this is the order in which the 

 colours occur, the red ring must necessarily have a less dia- 

 meter than the green, and the green than the violet; and 

 when the prism, set in a horizontal position, separates those 

 colours from each other, the sides of the resulting spectrum 

 ought not to be parallel but inclined to one another, the 

 breadth being least in the red, and increasing as we pass to 

 the violet end. This increasing breadth proves that the con- 

 stituent coloured shells of the flame envelope each other, the 

 violet being outermost and therefore broadest. This valuable 

 indication would be wholly lost if the slit was vertical. 



This being understood, I may illustrate the facts now to be 

 brought forward by an example of the prismatic analysis of a 

 horizontal element of the flame of a spirit-lamp ; it being un- 

 derstood that the prism is at its angle of minimum deviation, 

 and the spectrum seen through the telescope. All the pris- 

 matic colours in their proper order are visible, the sides of the 

 spectrum not being parallel, the inclination being quite rapid 

 toward the red extremity, the rays of which come from the 

 interior of the flame where the diameter is less. Mere in- 

 spection is sufficient to show the rapid approach of the red 

 sides to each other ; and I satisfied myself that, even in tlie 

 more refrangible regions, there is the same want of parallelism, 

 by rotating the telescope on its vertical axis, so that the vertical 

 wires in its eye-piece might coincide with first one and then 

 the other side of the spectrum. It will be understood that I 

 took the proper precaution not to be deceived by a partial 

 want of achromaticity in the telescope, which might have led 

 to a mistake. 



But, further, the yellow space of such a spirit-flame spec- 

 trum is crossed by a bright fixed line, — Sir David Brewster's 

 monochromatic ray. It is a beautiful example of the principles 

 just pointed out in tiiis method of horizontal analysis, being 

 of much greater width than the rest of the spectrum, and re- 



