340 History of Mechanical Inventions and 



Societies both of London and Edinburgh, and a particular account of it 

 may be expected in the Transactions of one or other or both of these learn- 

 ed bodies. 



Mr Ritchie's photometer is (like that originally proposed by Lambert in 

 his Photometria in 1760) a thermometer which measures the heat produced 

 by absorbed light, and is, therefore, liable to all the objections which 

 Lambert and others have urged against the instrument as a measurer of 

 different kinds of light. Our readers are well aware that Mr Leslie 

 brought forward Lambert's photometer as an invention of his own, and 

 proposed it as an instrument for measuring every kind of light, the light 

 of the sun, (not the light of the moon,) the light of the sky, the light of 

 snow, and the light of coal-gas, nay, even the light of the coloured spaces 

 Of the spectrum. Now, it is an admitted fact, that the red space in the 

 spectrum (to say nothing of the rays beyond the red) shows greater heat 

 by the thermometer than the yellow space, while the yellow space is far 

 more luminous than the red. The thermometrical photometer, therefore, 

 is incapable of measuring different kinds of light. Mr Ritchie, with a de- 

 gree of candour not inferior to his ingenuity, admits at once that his 

 photometer, which is fifteen or twenty times more sensible than Mr Leslie's, 

 is positively incapable of measuring or comparing any other lights but 

 those of the same kind ; and he is now convinced, that it will not measure, 

 except by a rude approximation, the relative illuminating powers of oil and 

 coal-gas. 



The principles on which this photometer depends are, that radiant heat 

 does not pass through thick plates of glass, but is conducted through them 

 in the same manner as through opaque bodies ; that light expands in the 

 same manner as heat the substances that absorb it; and that the intensity 

 of light varies inversely as the square of the distance. 



The Photometer shown in Plate IV. Fig. 10, consists of two broad flat cy- 

 linders, A, B, placed parallel to one another. These cylinders are air tight, 

 being closed at their outer sides by a thick disk of glass, while their inner 

 sides and their circumference consist of copper or brass, or sheet iron. 

 Parallel to the glass disk there is stretched across each cylinder a disk of 

 black paper. The cylinders are connected interiorly by a bent tube, DE, 

 containing a small quantity of coloured liquor ; and a scale is placed be- 

 tween the two branches of the tube which are vertical. By exposing the 

 glass faces of the cylinders to two lights, the light falling on the disks of 

 paper is absorbed ; the air within the cylinders is heated proportionally, and 

 the relative expansions indicated by the motion of the liquor towards 

 the weaker light. This instrument is said to be sensible to the light of a 

 candle at the distance of twenty or thirty feet, whereas Mr Leslie's shows 

 total darkness at the distance of three inches. 



In reference to the application of this instrument, Mr Ritchie remarks, 

 •' It would be a vain attempt," says he, " to endeavour, by the aid of this 

 photometer, or that of Professor Leslie, to ascertain the value of the illu- 

 minating powers of oil and coal-gas, since the qualities of the gas are es- 

 sentially different. When the flames are nearly similar, as in the case of 



