Ferret — Measures of Intensity of Solar Radiation. 379 



same, and formula were given for computing the true relative 

 intensities from the observed absolute temperatures and their 

 differences. Several other forms of apparatus of the same 

 nature might be here named, used for the same purpose, and 

 all affected similarly by the same cause of error. In all these 

 instruments it is assumed, in accordance with what is called 

 Newton's law, that the rate of radiation of any body, and so 

 of its rate of cooling by radiation, is proportional to the dif- 

 ferences of its temperature and that of the surroundings, what- 

 ever the absolute temperature may be, and so that these 

 differences are true measures of the intensity of the radiation 

 by which they are maintained. But this so-called law of New- 

 ton's was merely a first suggestion before experiments had been 

 made to test its accuracy, and it is now known that it does not 

 hold, with any degree of accuracy, through a range of 10° C. 

 It is therefore remarkable that certain physicists, even in recent 

 times, have extended this law up to the high temperature of 

 the sun, and have thus obtained temperatures for that lum- 

 inary, ranging from 10,000,000° C. down to much lower, but 

 still very extraordinary temperatures, all based upon the 

 assumption that Newton's law holds through so great ranges 

 of temperature. 



A modified form of the thermopile, called the registering 

 actinometer, has been used for several years by M. A. Crova 

 to determine the relative intensities of solar radiation. It is 

 composed of two parallel disks, each one consisting of two 

 metallic plates soldered together under pressure of ■£$ of a 

 millimeter in thickness and 0'015 m in diameter, constituting a 

 thermo-electric element fastened in a thin tube of brass ; one 

 of the junctions is in the dark, and the other receives a pencil 

 of solar rays falling normally upon the blackened surface in 

 the axis of the tube, which is provided with live diaphragms 

 of aluminum of progressive decrease of opening down to the 

 last, which is 4 mm in diameter. The opening of the tube is 

 kept, by means of clock-work, accurately directed toward the 

 sun during the day, and the amount of deflection of the gal- 

 vanometer needle at any time, represented by a continuous 

 curve of the registering apparatus, is taken as a true measure 

 of the intensity of solar radiation. The following are some of 

 the results obtained by this apparatus (Com'pt. Rend., ci, p. 

 418, August, 1885): " At sunrise the radiation increases with 

 rapidity until nine or ten o'clock, an epoch at which it always 

 attains its maximum, then it oscillates rapidly from one part to 

 .the other of its mean value, which diminishes on attaining a 

 minimum at the time of greatest temperature, it then increases 

 until about four o'clock without ever attaining the maximum 



