402 
Proceedings of the Royal Society of Edinburgh. [Bess. 
photometric measurements. He applied this method to fluorescein under 
various conditions, and his results were wholly negative. 
The object of the present article is to describe an attempt to detect 
fluorescence absorption in the case of fluorescein and uranium glass. Two 
different kinds of uranium glass were used. The method employed was 
in principle the same as the method of Nichols and Merritt, though the 
experimental arrangement was somewhat different. The results were 
wholly negative, and attention is directed to two causes of error which 
may not have received sufficient care, one of which produces a spurious 
effect, having one of the properties claimed for fluorescence absorption by 
Nichols and Merritt. It is true that the analogous reversal of a spectral 
line cannot be produced very easily ; but, making every allowance for the 
uncertain nature of the phenomenon studied, the author inclines to the 
belief that all cases of fluorescence absorption hitherto obtained have been 
due to systematic errors in the photometric arrangement. 
The spectrophotometer * used has already been described elsewhere, 
and proved admirably adapted for the purpose. It consists of a spectro- 
scope before the slit of which is fitted a polarising prism specially designed 
to divide the field into two halves which are polarised at right angles to 
one another, and which touch one another without overlapping. The 
fields are matched by means of a nicol eyepiece. Light enters the special 
prism by two apertures, one for the upper and one for the lower half of 
the field, and in obtaining the transmission curve of a piece of coloured 
glass it is put in turn before each aperture, both apertures being illumi- 
nated by a portion of an incandescent mantle 15 cms. away. When a 
self-luminous piece of glass is used close up to the aperture, if the light 
emitted by the glass is comparable in intensity with the light transmitted 
by it, it cannot be placed before either aperture, but must be placed 
before the lower beam e, f in fig. 2 in the article above referred to. If 
it is placed before the upper one, the light in the upper half of the 
field will not be quite plane polarised, for some of the rays diverge at 
too great an angle. 
The transmission curve of the substance investigated was first of all 
determined in the ordinary way, the substance being placed in turn before 
the two apertures, just as if it were an ordinary piece of glass. The 
fluorescent light can be neglected in comparison with the transmitted 
light. 
Then the arrangement shown in the diagram 1 was used, and the 
* R. A. Houstoun, “A New Spectrophotometer of the Hiifner Type,” Phil. Mag., 
Feb. 1908. 
