184 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



the same time to the eye, and we diminish the brighter one 

 until it becomes of equal intensity to the feebler. The quan- 

 tity of this diminution measured as a fraction of the primi- 

 tive intensity expresses the comparative brightness of the 

 two lights. In place of estimating directly the equality of 

 the brightness of the two images, we can oppose them one 

 to the other, giving birth to phenomena such as will render 

 their perfect equality more sensible for example, by trans- 

 forming and transmitting the inequality of intensity into the 

 production of color. The photometers of comparison which 

 possess the greatest perfection can perhaps be called pho- 

 tometers by opposition; such are those of Wild, Bunsen, 

 Dove, and one of Arago's photometers. The essential part 

 common to all photometers by comparison and by extinc- 

 tion is that which is designed to diminish the intensity of 

 the light, under the condition that the quantity of this dimi- 

 nution shall be exactly measurable. To this end, recourse 

 has been had to the eight following methods: First, the ab- 

 sorption of the light by an apparently transparent medium 

 of variable thickness. Second, reflection from a polished 

 surface at a variable angle. Third, reflections from one, 

 two, three, or more surfaces respectively, at an invariable 

 angle. Fourth, the reduction of the intensity by a deviation 

 by means of reflection of a portion of the light. Fifth, re- 

 duction of the intensity by two polarizing systems or planes 

 of polarization, by inclining them to each other at an invari- 

 able angle. Sixth, reduction by bifurcation of the ray in a 

 double refracting prism. Seventh, reduction by the increas- 

 ing separation of the rays of a conical pencil. Eighth, ex- 

 clusion of a portion of the beam of light which enters the 

 pupil of the eye. This exclusion may be accomplished by 

 means of a diaphragm placed either near the eye or in front 

 of the objective of the telescope, or within the terrestrial 

 telescope at the place occupied by the small diaphragm of 

 the quadruple ocular. Of all the combinations that we have 

 enumerated, Thury has chosen one of the most simple, viz., a 

 photometer having a variable diaphragm and reflecting mir- 

 rors. His instrument is adapted to a four- and- a-half-inch 

 refractor of excellent defining power, and has since 1868 

 been applied especially to the nebula) and the components 

 of double stars. The photometric system adopted is, there- 



