Scientific memoirs : being experimental contributions to a knowledge of radiant energy / by John William Draper.

  • Draper, John William, 1811-1882.
Date:
1878
Catalogue details

Licence: Public Domain Mark

Credit: Scientific memoirs : being experimental contributions to a knowledge of radiant energy / by John William Draper. Source: Wellcome Collection.

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    that centre and tP into another focus, and by the thermo- pile, or any other suitable means, measuring the heat of these foci. Such is the method I have followed in obtaining the measures now to be presented: but before giving thera, there are certain preparatoiy facts which I wish to sub- mit to the consideration of the reader. (1.) In the mode of experiment hitherto adopted, no special care has been taken to ascertain with accuracy the position of the extreme red, yet that is held to be the point from which, on one side, we are to estimate the in- visible, and, on the other, the visible spectrum. Differ- ent persons, perhaps because of a different sensitiveness of their eyes, will estimate that position differently. The red light shades off gradual!}^—it is almost impossible to tell when it really comes to an end. A linear thermo- pile, such as is commonly used, is liable, under these cir- cumstances, to give deceptive results, and any error in its indications counts in a double manner. It not only di- minishes the value of one spectrum, but it adds that dim- inution to the value of the other. The force of this re- mark will be understood by considering the best experi- ments hitherto made on this subject, those of Dr. Tyndall, as related in his Heat a Mode of Motion (London edi- tion, 1870, p. 420, etc.). In the case of the electric light, the result yielded by those experiments was that the heat in the invisible is ei^ht times that of the visible region. But had there been an error in estimating the position of the extreme red by only two millimeters, so much would have been taken from the invisible and add- ed to the visible that they would have been brought to equality, and then the slightest turn of the screw that carried the pile towards the dark space would have given a preponderance to the visible. It is obvious, therefore, that there cannot be certainty in such meas-