218 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



work movement. One of the great difficulties in the manufacture of 

 siderostats is the plane mirror, as its surface must be brought to the 

 greatest possible geometrical perfection. This is the essential difference 

 between a heliostat and a siderostat. In the heliostat, the principle 

 is to obtain a constant direction for the. reflected rays ; as it is light 

 which is studied, not the luminous source itself, therefore it matters 

 little if this is represented exactly or not. The siderostat, on the 

 contrary, must give an exact image of the heavenly bodies. This 

 difficult problem of the realization of an optical plane was solved 

 by Le'on Foucault by the use of a method which this ingenious 

 physicist, whose early death is greatly deplored, imparted to one 

 of his friends, M. Ad. Martin. 



M. Wolf has summed up the advantages of the new instrument as 

 follows unfortunately, at the time we write, it has not been tested 

 by any observations of sufficiently long standing : " There is not an 

 observer who has not had to contend with the difficulties presented 

 by the adaptation to an equatorial of a large spectroscope, photo- 

 graphic camera, projection apparatus, or photometric apparatus. All 

 these difficulties disappear by the use* of a siderostat. Laboratory 

 instruments, whatever their weight, size, and form, are placed in the 

 focus of the telescope as before the mirror of the camera obscura, 

 and the astronomer studies the light of all the stars under the same 

 conditions as the physicist has studied the light of the sun. By 

 this means experiments, which up to the present time have been 

 almost impracticable, may now be easily made ; particularly those 

 which require perfect stolidity of the instrument for measurement, 

 such as the determination of the exact position of the lines of the 

 spectrum, and the displacement of those lines, photometric measure- 

 ments, &c. 



" The mirror of the siderostat, tried with the excellent telescope 

 made by Cauchy, of sixteen centimetres aperture, which belongs to 

 the Paris observatory, with a magnifying power of from 100 to 300 

 times, does not produce any distortion of the rays proceeding from 

 the star at an angle of more than 45." 



The loss of light occasioned by the reflection is slight. According 

 to Foucault's experiments, it does not exceed in polished silver 

 mirrors T ^ of the incident light. Besides, the polish lasts a very long 

 time, and, as the re-silvering is easy, the mirror can be renewed as soon 



