162 
On the Connection between 
convenient to examine them in the small glass cells used in all my 
chromatological inquiries, cut from barometer tubes, having a 
length of -J inch and an internal diameter of |th inch. The 
spectrum of the light of fluorescence depends a good deal on the 
kind of illumination. Of course transmitted light must be entirely 
excluded, and the solution illuminated by strong light thrown 
perpendicular to the line of vision. For this purpose I use a bulls- 
eye condenser, which for the study of some substances is better if 
made of quartz, but for very many may be of glass. I then con- 
centrate direct sunlight on the uppermost part of the solution in 
the cell, so as to get very bright fluorescence, and at the same time 
to avoid letting the light of fluorescence pass through any such 
material thickness of the solution that it would lose any portion by 
simple absorption. The eflect of this may easily be seen by con- 
centrating the sunlight on the lower part of the cell, and allowance 
made for any slight action due to this cause. Having observed the 
spectrum of the light of fluorescence, it is easy to examine that of 
transmitted light, by removing the condenser, screening off the 
side light, and reflecting light up through the solution by means of 
the usual mirror below the stage of the microscope. 
Whilst thus describing the general phenomena of fluorescence, 
it would, I think, be well to allude to what looks like it, but is 
entirely spurious. There are some cases in which we appear to 
have a perfectly clear coloured solution with strong fluorescence ; 
and yet the substance is not really dissolved, but disseminated 
through the liquid in such extremely minute particles that they 
do not subside, and can be seen only by high magnifying powers, 
moving about tumultuously in currents due to slight alterations in 
temperature. On illuminating such a liquid by concentrated sun- 
light, passed through a solution of some salt of didymium, the 
spectrum shows all the bands due to this substance ; whereas, if the 
liquid were quite clear and possessed genuine fluorescence, no trace 
of the didymium bands would be seen. The existence of some true 
fluorescence along with much spurious, due to minute disseminated 
particles, may be recognized by the presence of some of the 
didymium bands, and the comparative absence of others from those 
parts of the spectrum where the light of true fluorescence exists. 
By adopting these methods of observation, I have found that in 
the case of some substances the light of fluorescence does contain 
rays of greater refrangibility than those most readily absorbed by a 
dilute solution, and extends from the red end a little beyond the 
centre of the main absorption band. This is, however, only when 
the illumination is strong and near the upper surface, and, if 
weaker or lower down, so as to increase the effect of absorption, the 
spectrum of the light of fluorescence extends only to the centre of 
the mam absorption band, or may even stop considerably short 
