THE 



AMERICAN 



JOURNAL OF SCIENCE, &c. 



Art. I. — Description of the Tithonometer, an instrument for 

 measuring' the Chemical Force of the Indigo-tithonic Rays ; 

 by John W. Draper, M. D., Professor of Chemistry in the 

 University of New York.* 



I have invented an instrument for measuring the chemical 

 force of the tithonic rays which are found at a maximum in the 

 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. 



Impressed with the importance of possessing for the study of 

 the properties of the tithonic rays some means of accurate meas- 



* From the London, Edinburgh and Dublin Philosophical Magazine and Journal 

 of Science, for December, 1843. 

 Vol. xlvi, No. 2.— Jan.-March, 1844. 28 



