1826.] • of Photometry, with Remarks. 377 



however, one or two particulars which it certainly will serve to 

 exhibit. 



For example, it will serve to show whether there be in the 

 spectrum formed by a prism of any given substance any exterior 

 heat beyond the visible rays : that is to say, any heating powers 

 of a nature transmissible through glass, and acting on the black 

 bulb. It will not measure the quantity of such eftect, nor deter- 

 mine the point of maximum ; for in this case, there is an addi- 

 tional source of fallacy besides those acting in other cases : this 

 is the circumstance that when placed on or near the boundary 

 of the spectrum, so that only a part of the bulb is within it, or 

 that rays of different powers are acting on different parts of the 

 bulb, the balance of absorption and radiation of heat is greater in 

 favour of radiation than in those positions where the whole bulb 

 is equally immersed in the same ray. The effect of expansion in 

 the glass is also different : so that, independently of other sources 

 of error, such measures are not comparable with those in other 

 parts of the spectrum. These circumstances, if not sufficiently 

 attended to, become sources of error to a considerable amount, 

 and in some instances may lead to the most contradictory con- 

 clusions.* At all events, in cases where the object is merely to 

 determine the existence of a particular heating effect, not to 

 measure its magnitude, this instrument may be found applicable, 

 or where only a rough estimation is required, or merely to observe 

 whether a considerable increase or decrease of effect takes 

 place, it may, with proper precaution, be found useful. 



In Mr. Ritchie's second paper (Edin. Jour. No. 5, p. 107), he 

 gives the method used by Mr.Herschel for obtaining an accurate 

 measure of the heating power of the sun's rays. This method 

 was described in a letter from Mr. H. which formed part of a 

 communication from Mr. R. read before the Royal Society in 

 1825, but has not, that I am aware, been published any where 

 but in the paper above referred to. It consists in exposing in a 

 glass vessel or large thei-mometer, a deep-blue liquid for a given 

 time to the direct rays of the sun : noting the increase of tem- 

 perature, which is purposely rendered but small by properly 

 adjustingthe capacity of the instrument: then shading the sun's 

 direct rays, and leaving it exposed for an equal time to the free 

 influence of all other heating and cooling causes, radiation, 

 conduction, wind, Sec. and again noting the effect of these. 

 The same difference of these, according to their signs, is the 

 effect of mere solar radiation. Dividing this by the time of 

 exposure, the momentary effect or differential coefficient, which 

 is the true measure of the intensity of radiation, is obtained. 



• In particular I allude to some results whir li have fallen witliin my own experience. 

 I obtained sometime ago some apparently sinjjular indications rcspectinj; the heating 

 power of the solar spectrum, and cannot at present perceive to what source of fallacy thejr 

 van be traced. I shall probably take a future opportunity of recurring to this subject. 



