C. GENERAL PHYSICS. 185 



fore, that of the visual photometer by extinction. The en- 

 feebling of the light is obtained by reflection from one or 

 more mirrors situated between the objective and the ocular, 

 and by a diaphragm having a variable opening placed in 

 front of the objective. This diaphragm is composed of six- 

 teen thin rectangular plates, sliding simultaneously and uni- 

 formly each in the direction of its length, and the direction 

 of the radius that passes through the centre of the objective. 

 The polygon of a variable diameter and symmetrical form 

 is the real aperture of the objective. When the aperture of 

 the telescope is diminished too much, the dimensions of the 

 false disks of the stars increase, and the diffraction rings that 

 surround the false disk become so modified that the con- 

 ditions of visibility are no longer the same for two stars 

 viewed with very different apparatus. It is necessary, there- 

 fore, to correct this source of error by diminishing the bright- 

 ness of the brightest stars, not by contracting the aperture, 

 but by introducing the use of mirrors. A comparative table 

 is given, showing the relative effects of mirrors and dia- 

 phragms. Two classes of scales have been adopted in ex- 

 pressing the orders of brightness of the stars. The photomet- 

 ric scale of Sir John Herschel was based upon the simple 

 fact that the intensity of light diminishes as the square of 

 the distance. The brightness of the stars belonging to the 

 first, second, third, etc., magnitudes on his scale w T as there- 

 fore respectively one quarter, one ninth, one sixteenth, etc. 

 The system more generally followed is such that the bright- 

 ness of a star of any order is always a certain constant frac- 

 tion of the brightness of a star of the next succeeding order, 

 so that the arithmetic series of magnitudes corresponds to a 

 o-eometrical series of intensities. The constant ratio in this 

 system would naturally be so chosen as to change as little 

 as possible the magnitudes that have been somewhat arbi- 

 trarily assigned to the stars by many generations of astrono- 

 mers. The actual photometric series of Sir John Herschel 

 accords remarkably well with the ordinary scales of magni- 

 tudes, if we simply multiply his magnitudes by 1.41, and 

 take for the unit of intensity a star equal to that of Alpha 

 Centauri. But the photometric scale of this astronomer of- 

 fers grave inconveniences, which have hitherto prevented its 

 being adopted. The smallest star visible in the twenty-foot 



