FOURTH SERIES. METALLIC GRATINGS. 



15 



glass, gave some countenance to this experiment. I was not unaware that dif- 

 fraction spectra are produced, not by a parallel beam of light, but by a picture, 

 formed of a distant luminous point. Still, though the ground or field illuminated 

 b} r parallel rays passing through a grating must evidently have a uniform tint, it 

 does not appear absurd to suppose that that tint may be different from white. 

 Nor does this question appear to have occurred to mathematicians or optical 

 writers, until the problem presented itself to me in the course of this investiga- 

 tion. 



32. With such wire-gauze as I could easily procure, I failed in obtaining any 

 peculiarity of action as relates to heat from different sources ; and farther, the 

 quantity of heat intercepted by the metallic grating appeared to be nearly, or 

 exactly, proportional to the surface of the opaque portion of the screen. Think- 

 ing that perhaps finer gauze than that I used (60 wires to the inch) might pro- 

 duce the desired effect, I obtained, through the kind assistance of Sir JOHN ROBISON 

 and M. LEONOR FRESNEL, the finest manufactured in Paris, going as high as about 

 160 per inch. In general my first results were confirmed, viz. (1.) that the pro- 

 portion of heat stopped is irrespective of the source ; (2.) that it is to the incident 

 heat as the area of the wires is to the area of the surface. It must be observed, 

 however, that the determination of this latter proportion with extreme accuracy by 

 an examination of the grating, is not so easy as might at first sight appear. When 

 the roire is fine compared to the interstices, the interstices are pretty nearly rec- 

 tangular and equal-sided. But this is not the case in most manufactured wire- 

 gauze. One set of wires is nearly parallel and straight, but not so the set inter- 

 laced with the former, which do not generally make their intersections at right 

 angles, and hence, universally, the interstices are somewhat smaller than a calcu- 

 lation proceeding upon the number of wires per inch, and their diameter would 

 give. Distrusting my own observations, I put three specimens of wire-gauze into 

 Mr JOHN ADIE'S hands, requesting him to determine the mean diameters and in- 

 tervals of the wires. With a very accurate micrometer he determined 14 values 

 for each of these quantities in both directions. From these data the proportion 

 of the Interstices to the whole Surface of each grating is easily calculated, and 

 the results are given below for three sorts of gauze of which I had previously de- 

 termined the permeability for heat. 



Micrometric Measurement of Wire-Gauze. Unit of Measure = 



inch. 



