438 
POPULAR SCIENCE REVIEW. 
lengths and frequency of vibrations, this measures amplitudes on the inten- 
sity and brilliancy of light. It depends on the optical principle that light 
becomes invisible when in the presence of light sixty-four times more 
brilliant. 
His mode of operating is as follows : — 
Remove from the common three-tubed spectroscope its scale tube, and 
place against the aperture into which it was screwed a piece of glass ground 
on both sides. In front of this arrange an ordinary gaslight attached to a 
flexible tube, so that its distance from the ground glass may be varied at plea- 
sure. On looking through the telescope, the field of view is seen uniformly 
illuminated, the brilliancy depending on the distance of the gaslight, accord- 
ing to the ordinary photometric law. This is called the extinguishing light. 
If it be put out for a moment, and the luminous flame of the Bunsen burner 
so arranged that its spectrum is seen, on relighting it a spectrum is seen in 
the midst of a field of light, the brilliancy of which can be varied at plea- 
sure. As the intensity of the latter is increased, the violet first disappears, 
then the other more refrangible colours in their descending order, till at 
length the red alone remains. The yellow never stands out conspicuously, 
as might have been expected. This is scarcely consistent with the opinion 
that the yellow is the brightest of the rays. Dr. Draper, therefore, thinks 
that the luminous intensity of the coloured spaces has a relation to the com- 
pression or condensation that the prism is impressing upon them. To 
diminish the intensity of the extinguishing light, an exceedingly thin film of 
silver was deposited on the face of the prism which acts as reflector. 
This, though quite transparent to the transmitted rays, increased by metallic 
reflection the extinguishing rays. 
The same estimate was made more simply with sunlight, by introducing a 
beam from a Heliostat, and throwing its spectrum on a screen. The window- 
shutters were then more or less opened to admit ordinary light. When the 
shutters were wide open, the spectrum was quite obliterated ; on gradually 
closing them, the red region first came into view, the other colours following 
in order of refrangibility. Irrationality of dispersion in various kinds of 
glass did not seem perceptibly to influence the results, nor did differences in 
the eyes, the age, or the visual powers of different observers. 
Dr. Draper saw at once in the dispersion-spectrum a means of testing his 
views as above stated. Using a grating, and the spectrum of the first order, 
as the force of the extinguishing illumination increased, all the coloured 
spaces yielded equally and simultaneously, re-appearing in a similar manner. 
The same happened with solar as with gaslight. 
Light from Thermo-Electricity. — M. Clamond appears to have overcome 
the difficulty of producing a battery of this nature fit to produce the electric 
light. It is formed of iron as the electro-positive element, and of an alloy 
of antimony and zinc for the negative. They are arranged in a circular 
form, heated in the interior. The latest form consists of a number of pieces 
of cast irony named the collector , so arranged that heated air can circulate 
between them. A large surface is thus exposed to the heat, which the iron 
collects, and communicates to the couples. The Diffuser is the outside of 
the apparatus, and is made of sheets of metal. The thermopile itself is 
placed between these two, and is so arranged that the junctions of the 
