24 Mr. Robert Sabine on a 



normally upon the face of the wedge. The light then passes 

 through and is weakened by the wedge, then through the strip 

 of opal glass, through the slit behind it into the interior. 

 Here it is incident upon a small mirror made of a strip of sil- 

 vered microscope covering-glass, placed at an angle of 45° to 

 the axis of the tube, and is thence reflected to the eye of the 

 observer. The other end of the photometer, close to the bright 

 part of the paraffin-flame, is furnished with a cap containing 

 a circular diaphragm disk with three different apertures, either 

 of which can be inserted by simply turning the disk. The 

 little mirror is supported from the face of the constantly illu- 

 minated opal pane in such a way that the support is hidden 

 from the observer by the mirror. The object of this arrange- 

 ment is to ensure the observed surfaces whose illuminations 

 are to be compared being in juxtaposition without any appa- 

 rent division-line or separation. It is well known* that when 

 the two surfaces to be compared are at a perceptible distance 

 apart, as in Rumford's photometer, no certainty of accurate 

 measurement is possible. The method of supporting a small 

 mirror in the centre of the field of comparison in the way de- 

 scribed is very satisfactory. The glass of the mirror being 

 extremely thin, I find with a moderate illumination, and when 

 the light reflected by the mirror is of the same colour as that 

 of the field, the mirror sometimes entirely vanishes from sight 

 at the moment of balance. 



The distance at which the light to be observed is placed 

 from the photometer varies, of course, according to conveni- 

 ence; but where it is optional, I prefer for all lights under 

 1000 candles a constant distance of 1 metre. Stronger lights 

 become inconvenient to the observer if placed so near; and in 

 that case I increase the distance accordingly, while of course 

 at the same time increasing the range of the photometer. 



The degree of illumination of the field of comparison is 

 adjustable by means of the diaphragms. The smallest dia- 

 phragm illuminates the field so that the light of a single 

 candle can be conveniently balanced. The next diaphragm 

 exposes an area about eight times as large of the bright por- 

 tion of the paraffin-flame, and the largest diaphragm about 

 sixty-four times ; that is to say, it has about eight times the 

 diameter of the smallest diaphragm. The resulting degrees of 

 illumination of the field can be thus calculated as well as 

 compared by actual observation. Let the illumination of the 

 field with these three diaphragms be respectively X 1? X 2 , X 3 . 

 Of the light which falls upon the outer face of the wedge a 



* Herschel, ' On Light/ p. 29; Helmholtz, Physiohgische Optik, p. 329. 



