Heat ill the Spectrum. 113 



not answer ; the surface so produced is too glossy and reflect- 

 ing-. The plan I have found best is to take a glass tube | an 

 inch in diameter and 6 inches long^ open at both ends, and use 

 it as a chimney. A piece of camphor being set on fire at the 

 lower end_, and the face of the pile to be blackened being held 

 for a moment at the upper, it is covered with a dense black film 

 without any risk of injury to the pile. Even at the best, when 

 this has been done, there is an unavoidable source of error in the 

 want of perfect blackness of the lampblack. It is sufficient to 

 inspect the face of the pile when receiving rays from the concave 

 mirror to be satisfied how large a portion of light is reflected. 

 The experiments of Dr. Tyndall show that this substance trans- 

 mits a considerable percentage of the heat falling on it. Its 

 quality of transmitting light is well known to every one who has 

 looked at the sun through a smoked glass. 



The galvanometer I have used is calibrated according to the 

 nsual method ; the numbers given in this memoir do not repre- 

 sent the angles of deflection, but their corresponding forces. 



The proper position of the intercepting screens h, i can often 

 be verified with precision by looking through a blue cobalt glass. 

 This glass insulates a definite red, an orange^ and a yellow ray 

 in the less-refrangible regions, and then, commencing with the 

 green, gives a continuous band to the end of the violet. Its red 

 ray begins at the less-refrangible end of the spectrum, and ends 

 near C. It includes the fixed lines A, B, C. Its orange ray 

 lies wholly between C and D, including neither of those lines. 

 Its yellow ray begins near 5894, and ends about 5581 ; the line 

 D is therefore near its point of beginning. Its end is about 

 halt\vay from D to E. The remaining continuous band begins 

 about 51.25 ; it therefore includes the lines E, ^, F, G, H. I 

 have found this glass of much use in determining how far the 

 screen i has been pushed. It is convenient to select a light kind 

 of it ; and by looking through one, two, or three pieces the depth 

 of colour can be regulated at pleasure. 



The optical train which has thus acted on the sunbeam under 

 examination is therefore (1) the sun's atmosphere, (2) the earth's 

 atmosphere, (3) the heliostat-mirror of speculum-metal, (4) the 

 prism, (5) the concave mirror of silvered glass, (6) the black- 

 ened face of the thermopile. 



Results obtained by the Apparatus. 



We are now ready to examine the results which this optical 

 apparatus yields, it having been, of course, previously ascertained 

 that the reflecting band of the concave mirror dd is sufficient to 

 receive all the radiations coming from the prism, and that none 

 arc escaping past its edges. 



PhiL Mag. S. 4. Vol. 44. No. 291. Au(/. 1872. I 



