C. GENERAL PHYSICS. 183 



of the distribution of the stars in space, and the gradual mod- 

 ifications that celestial bodies undergo in their own nature, 

 are intimately connected with the intensity of the light we 

 receive from them or that they send out; but it would be 

 impossible to find a collection of photometric observations 

 sufficient to serve as a basis for safe deductions. Either the 

 accuracy of the observations is too small, or there are not 

 enough of them. The observations that Ave have are due, 

 for the most part, to experienced observers, and the differ- 

 ences between their methods of research fully explains the 

 diversity of their results. In order to make their observa- 

 tions comparable with each other, and to eliminate causes of 

 error peculiar to each method, it is necessary to institute 

 comparative observations by making use of each of the 

 methods employed hitherto in photometry. The results of 

 such an investigation, which has already been commenced in 

 Germany, will probably be, first, a general accordance of the 

 figures obtained by different methods, sufficient to give con- 

 fidence in their exactness. Second, the knowledge of the 

 means proper to bring about such an accordance ; that is to 

 say, a knowledge of the universal corrections and of the im- 

 provements necessary to be introduced into the apparatus, 

 and the methods of employing it. Finally, Ave shall know 

 which of the photometric methods permits the greatest de- 

 gree of exactness, and which offer special advantages. In 

 general the photometers hitherto employed may be divided 

 into two categories : First, those where the object affected by 

 the light is the eye itself. Second, the physical and chem- 

 ical photometers, where some inert body is modified by the 

 light. Of these latter, that of Leslie and the photographic 

 photometers are instruments especially adapted to meas- 

 ure separately the intensity of different kinds of radiations. 

 Visual photometers are divided into two classes. In one 

 Ave diminish the brightness of the light until it disappears 

 from the sight, or, rather, until it becomes too feeble to en- 

 able us to distinguish certain definite details of objects, and 

 Ave then calculate the quantity of the diminution by know- 

 ing the methods employed to produce it. These are the 

 photometers of extinction; such are those of Arago, Xavier, 

 and Maistre. The second class of visual photometers is that 

 of comparison, Avhere the two lights present themselves at 



