124- Professor Forbes's Researches on Heat. 



gest inquiries of no small interest with regard to light, and 

 especially the phenomena of absorption. 



59. We have already (24-.) noticed the analogy which the 

 fact stated in the last article bears to the easier reflexion of 

 red than violet light from depolished surfaces, and in that 

 fact we find a confirmation of the application of the undula- 

 tory doctrine to heat, and of the opinion that the waves pro- 

 ducing heat are longer in proportion as the temperature of 

 the source is less. The phaenomena of transmission are more 

 obscure; they may be compared either to the diffraction, or 

 to the absorption, of light. 



60. The action of lines on polished surfaces, similar to 

 those used in many diffraction experiments, led to the in- 

 quiry (31.) whether the mean colour of light transmitted by 

 gratings was necessarily unchanged ? The question does not 

 seem to have occurred to any one to whom I have mentioned 

 it; and though the most likely result would seem to be, that 

 there should be no change, the grounds of such an a priori 

 opinion do not appear absolutely conclusive. Professor Kel- 

 land, however, has, I believe, first succeeded in integrating 

 the expression for the illumination of a screen placed behind 

 a grating of any kind (see Airy's Mathematical Tracts, page 

 328) on which a plane wave falls, and he informs me, that in 

 every case where the breadth of the interstices is any multiple 

 of the breadth of the wires or opake spaces, the intensity is 

 the same as if there were a diaphragm equal in size to the sum 

 of the interstices of the grating. 



61. This result (which seems quite sufficiently general for 

 our purpose) is so far confirmed by the absolute indifference 

 of metallic gratings to the quality of the incident heat. 



62. It remains, however, to be explained how furrowed 

 surfaces can act, except by intercepting, as an opake network 

 would do, a part of the heat. I cannot give an explanation 

 which appears full and satisfactory, but the condition of mica 

 split into thin laminas by heat, and producing the same effect, 

 may serve to guide us, perhaps, to something like the true 

 cause. 



63. A number of thin plates, of exactly uniform thickness, 

 would transmit a certain colour, and reflect the complement- 

 ary one. If there be a great preponderance of plates approxi- 

 mating to a certain thickness, and if the disproportion of the 

 lengths of the incident waves be great, a large proportion will 

 be in like manner transmitted, and the remainder stifled or 

 reflected. If this effect is not so frequently observed in bodies 

 mechanically separated into films as we might expect, this is 

 owing to the small range of length of wave in the visible parts 



