THE RADIATION OF THE SUN—ABBOT. 1438 
It is not necessary here to explain the details of the construction or 
the principle of it, more than to say that it is a particular form of 
spectroscope whose effect is to act as a screen to cut off all rays of 
the spectrum except the particular one which it is desired to observe. 
By the aid of this instrument the distribution of the gases of different 
elements over the sun’s disk has been investigated, notably of the 
gases hydrogen and calcium. Plate 2, figure 1, shows a photograph 
of a portion of the sun’s disk as observed in hydrogen. The reader 
will note the very prominent detail which is shown by this illustration 
as compared with that shown by direct photography of the sun’s 
surface by a telescope as given in plate 1, figure 1. It was by the aid 
of the spectroheliograph that Slocum obtained the beautiful figure of 
the solar prominence given in plate 3. 
SOLAR ENERGY. 
We now turn from this general consideration of what may be 
seen on the sun by the aid of the telescope and spectroscope to a 
discussion of the quantity of energy which the sun sends out, the 
distribution of it among the different spectrum rays, and the rela- 
tions which it bears to the temperature of the sun, the temperature 
of the earth, and other terrestrial concerns. I said a little while ago 
that light is regarded as of the nature of a mixture of vibrations in 
the ether, which is supposed to be a substance existing in all space, 
including the interstices of the structure of the chemical elements 
themselves. Light is but one of the manifestations of radiation. 
It is merely that kind of radiation which is visible to the eye. Just 
as there are some sounds which are of too high pitch for the ear to 
hear, and some other sounds which are of too low pitch to distinguish 
as sound, so there are kinds of radiation which are of too short wave 
length for the eye to recognize as violet light, and others are of too 
long wave length for the eye to recognize as red light. Indeed for 
the longer wave lengths of radiation the substances of the eye are 
not transparent, so that even if the retina should be sensitive to these 
rays, they could not reach the retina to affect it. 
In this state of affairs it is necessary to proceed to the investiga- 
tion of the energy of radiation by means of another instrument in 
which the radiation is caused to be absorbed by a blackened surface, 
and thus to produce heat, and consequently a change of temperature 
of the absorbing substance. Radiation is not heat. Heat is a 
motion of the molecules of the material substance, but radiation is a 
motion of vibration in the ether, which is not regarded in the same 
category with ordinary chemical elements. Indeed, we may go a 
little further and make a classification of energy. Imagine a chest of 
drawers in which, as sometimes happens, the letters or other papers 
fall over the back of the drawers, as they are pulled out into the 
