14 



PROFESSOR FORBES'S RESEARCHES ON HEAT. 



Locatelli with Glass. 



Locatelli. 



51 



Dark Heat. 



Mica with rough surfaces, 45.5 51 31.5 



Denoting the original transmissions by 100, the diminished effect due to the rough- 

 ness of the surface will be represented by 



54 69 85 



demonstrating as clearly as possible that the stoppage is proportioned to the tem- 

 perature of the source of heat ; thus, whilst 46 per cent, of the first kind was ar- 

 rested by the roughness of the surface, only 15 per cent, of dark heat was stopped. 

 29. (3.) With regard to the third question, the action of a comparatively 

 small number of scratches on a polished surface, instead of a general diminution 

 of its polish, I proceeded thus : I caused a series of extremely minute lines to be 

 drawn mechanically with a diamond point, on a well polished surface of rock-salt, 

 so as to divide it into squares having one-hundredth of an inch for their side. A 

 similar plate was scored by fine lines in the same manner, parallel to one another, 

 and one two-hundredth of an inch apart. A portion of this second plate was 

 crossed rectangularly, by lines drawn at the same distance, so as to divide the 

 surface into squares four times smaller than in the first instance. These three 

 media gave the following results with two very different kinds of heat (December 

 0-11. 1839). 



For heat of 212 the per-centage was still higher, as will afterwards be shewn. 



30. Metallic Gratings. If the mere defect of transparency were the cause of 

 the peculiar action of scratched surfaces, we might expect that any opaque filaments 

 would act in the same way. Could we dispense with the medium altogether, and 

 employ a screen, which should have the qualities which we had artificially given 

 to the physical surface of the medium, we should evidently have advanced a 

 step in the interpretation of the phenomena. The action of grooved surfaces and 

 gratings upon light suggested so forcible an analogy, that before I was able to 

 procure the mechanically striated surfaces, described in the last article, I had 

 employed fine metallic wire-gauze as a diffraction-screen, hoping to obtain results 

 similar to those which I anticipated, and afterwards did obtain, by drawing fine 

 lines upon rock-salt. 



31. The fact that diffraction-phenomena in light, produced by gratings, are 

 wholly irrespective of the nature of these gratings, as, for instance, whether they 

 be formed of metal- wires, or mere lines drawn through a soapy film stretched on 



