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JOURNAL 
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OF SCIENCE, &c. 
Art. I.—Description of the Tithonometer, an instrument for 
measuring the Chemical Force of the Indigo-tithonic Rays ; 
by Joun W. Draper, M. D., Professor of Chemistry in the 
University of New York.* 
I wave invented an instrument for measuring the chemical 
‘indigo space, and which from that point gradually fade away to 
- each end of the spectrum. The sensitiveness, speed of action 
-and exactitude of this instrument, will bring it to rank as a means 
of physical research with the thermo-multiplier of M. Melloni. 
The means which have hitherto been found available in op- 
tics for measuring intensities of light, by a relative illumination 
of spaces or contrast of shadows, are admitted to be inexact.’ 
The great desideratum in that science is a photometer which can 
mark down effects by movements over a graduated scale. With 
those optical contrivances may be classed the methods hitherto 
adopted for determining the force of the tithonic rays by stains 
on Daguerreotype plates or the darkening of sensitive papers. 
As deductions, drawn in this way, depend on the opinion of the 
observer, they can never be perfectly satisfactory, nor bear any 
comparison with thermometric results. fe. 
Impressed with the importance of possessing for the study of 
the properties of the tithonic rays some means of accurate meas- 
3 ’ * From the London, Edinburgh and Dublin Philosophical Magazine and Journal 
__ f Science, for December, 1843. ; 
@ Vol. xzv1, No. 2—Jan.-March, 1844. 8 
