230 PEOCEEDINGS OF THE AMERICAN ACADEMY 



These, however, do not show the facts in so clear a manner. On re- 

 ceiving the impress of a solar spectrum they present patches of light 

 and shade irregularly distributed. Though in a general way they 

 confirm the statements made above, they do not do it sharply or 

 satisfactorily. 



Dubosc's specimens to which I have had access are enumerated as 

 follows: 1. Calcium violet; 2. Calcium blue; 3. Calcium green; 4. 

 Strontium green ; 5. Strontium yellow ; 6. Calcium orange. Restrict- 

 ing my observation to the space beyond the red, — which, as has been 

 said, presents a bright rectangle in the darkness, about as far below 

 the red as the red is below the yellow, — I found that this rectangle is 

 not given by 1 and 2. In 3 it is doubtful. In 4 it is quite visible, 

 and in 5 and 6 strikingly so. 



Is the blackening then due to heat ? That it occurs beyond the 

 violet, that is, beyond the lines H, seems to render such an opinion 

 doubtful, for it is commonly thought that the eflfect of heat is not rec- 

 ognizable there. And in the phosphorogenic spectroscope I have 

 used, the optical train, prism, lenses, &c., is of glass, which must of 

 course exercise a special selective heat-absorptiou ; but the traces of 

 this in the phosphorograph I could never detect. 



In the diffraction spectrum, I had attempted nearly forty years ago 

 to ascertain the distribution of heat (Phil. Mag., March, 1857), but 

 could not succeed with the experiment in a completely satisfactory 

 manner, so small is the effect. I exposed a tablet of luminous paint 

 to such a diffraction spectrum formed by a reflecting grating, having 

 17,296 lines to the inch, and was not a little surprised to see that 

 from the blue to the red end of the spectrum there is an energetic 

 extinction of the light, and darkness is produced. I repeated this with 

 other gratings, and under varied circumstances, and always found the 

 same effect. 



Now, considering the exceedingly small amount of heat available in 

 this case, and considering the intensity of the effect, is there not herein 

 an indication that we must attribute this result to some other than a 

 calorific cause ? 



I endeavored to obtain better information on this point by using the 

 rays of the moon, which, as is well known, are very deficient in heat- 

 ing power. Many years ago I had obtained some phosphorographs of 

 tliat object. With the more sensitive preparations now accessible, and 

 with a telescope 11 inches in aperture and 150 inches focus, there 

 was no difliiculty in procuring specimens about 1.4 inch in diameter. 

 These represented the lunar surface satisfactorily. At half-moon an 



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