PEOFESSOE STOKES ON THE LONG- SPECTEHM OE ELECTEIC LIGHT. 601 
A wide field of research was thus thrown open to any one taking the very moderate 
trouble attending the use of an induction coil. It remained to study the lines given by 
different metals and gases, and the absorbing action of various substances with respect to 
the invisible rays of different refrangibilities. 
Various observations were made from time to time in this subject. As regards the 
metallic lines, it is perfectly easy to view them at pleasure ; but to obtain faithful deli- 
neations of them is another matter. Even an accomplished artist would find difficulty 
in obtaining by mere eye-sketching a faithful representation of an object which requires 
to be seen in the dark. I tried different methods without being able to satisfy myself 
as to the accuracy of the drawings which could be thus obtained, and frequently thought 
of resorting to photography. 
Meanwhile the mode of absorption of the rays of high refrangibility by a good number 
of substances was observed. Nothing is easier, to a person provided with a cell with parallel 
faces of quartz, than to observe by means of fluorescence the mode of absorption of these 
rays by a given solution ; but to draw safe conclusions as to the optical character in this 
respect of the substance deemed to be in solution is not so easy as it might appear ; 
for the rays of high refrangibility are liable to be absorbed by an exceedingly small 
amoimt of an impurity which may chance to be present without the observer’s know- 
ledge. Thus I found that about a quarter of a square inch of clean filtering paper suffi- 
ciently contaminated the water contained in a small cell to interfere sensibly "with its 
transparency. Should the solution be transparent there would be no difficulty, for the 
effect of an impurity would not be to render transparent a solution which otherwise 
would be opaque. Should it, on the other hand, absorb the invisible rays, or some of 
them, with great energy, or in a peculiar manner, we might again conclude that we had 
obtained the true character of the substance deemed to be observed. The most remark- 
able example of this kind which I met with among inorganic colourless solutions was in 
the case of nitric acid and its salts, such as nitrate of potash, soda, ammonia, baryta, 
which absorb the rays of high refrangibility with great energy and in a peculiar 
manner, exhibiting a maximum of opacity followed by a maximum of transparency, 
beyond which the absorption becomes still more energetic than before. But if the 
solution should be found to absorb the rays of high refrangibility with only moderate 
energy, it would be left doubtful whether the observed absorption might not be due to 
some impurity ; and I did not see how this doubt could be solved otherwise than by a 
laborious system of recrystallizations. 
After having obtained these results, I found by conversation with my friend 
Dr. Miller that he also had been engaged at the same subject, working by photo- 
graphy, and had prepared a number of photographs of metallic spectra, and studied 
by the same means the absorption of the rays of high refrangibility by a great variety 
of substances, chiefly inorganic acids, bases, and salts, and the commoner organic bodies. 
Although a large part of the task which I had proposed to myself has thus been accom- 
plished in another way, there are many results which I have met with which are not 
4n2 
