J. W. Draper—Phosphorograph of a Solar Spectrum. 177 
be caused suddenly to receive a solar spectrum, it will instantly 
become brighter in the region of the less refrangible rays, as 
will plainly appear on the spectrum being for a moment extin- 
guished by shutting off the light that comes into the dark 
room to form it. If the light be re-admitted again and again, 
the like increase of brilliancy may again and again be observed, 
but in a declining way. Presently, however, the region that 
has thus emitted its light begins to turn darker than the sur- 
rounding luminous parts. If now we no longer admit,’‘any 
spectrum light, but watch the phosphorescent surface as its 
luminosity slowly declines, the region that has thus shot forth 
its radiation becomes darker and darker, and at a certain time 
quite black. The surrounding parts in the course of some 
hours slowly overtake it, emitting the same quantity of light 
that had previously been expelled from it, and eventually all 
becomes dark. 
ow, apparently, all this is in accordance with the hypothesis 
of the expulsion of the light by heat. There are, however, cer- 
tain other facts which throw doubt on the correctness of that 
explanation. 
On that hypothesis, the darkening ought to begin at the place 
of maximum heat, that is, when flint glass apparatus is used, 
below the red ray, and from this it should become less and less 
Intense in the more refrangible direction, But, in many experi- 
ments carefully made, I have found that the maximum of black- 
ness has its place of origin above the line D, and indeed where 
the orange and green rays touch each other. Not infrequently, 
in certain experiments the exact conditions of which I do not 
know and cannot always reproduce, the darkening begins at the 
upper confines of the green, and slowly passes down to beyond 
the red extremity; that is to say, its propagation is in the oppo- 
site direction to that which it ought to show on the heat 
hypothesis. 
Still more, as has been stated, there is a dark space above the 
violet. Now it is commonly held that in this region there is 
little or no heat. If so, what is it that has expelled or destroyed 
the light ? 
_ The experiments above referred to I made with the recently 
troduced luminous paint. It presented the facts under their 
simplest form. But I have also tried many other samples, for 
which I am indebted to the courtesy of Professor Barker of 
Philadelphia, Among them I may mention as being very well 
hown the specimens made by Dubosc, enclosed in flat glass 
tubes, contained in a mahogany case, and designed for illustra- 
ting the different colored phosphorescent lights emitted. They 
are to be found in most physical cabinets. These, however, 
do not show the facts in so clear a manner. On receiving the 
