1892.] 



Stellar Photometry. 



405 



The second portion of the inquiry was directed to the most suitable 

 means of comparing a star with the standard. Such a method must 

 provide for the estimation of the total luminous energy, irrespective 

 of the fact that in the case of a star the light practically emanates 

 from a point, whilst that from the standard emanates from a surface. 

 This fact precludes the use of the wedge photometer. 



The methods employed by the author were three in number. The 

 first was by means of reflection from a plane mirror mounted in front 

 of the telescope object glass, in such a manner that the light of one 

 of the radiants is seen by reflection through one half of the lens, 

 whilst that of the other is viewed through the other half, by direct 

 vision, both radiants being thus simultaneously seen in the same 

 field of view. When the two images are placed out of focus to an 

 equal extent, so as to be equal in size, comparisons of intensity can 

 be made. By this system it matters not whether the light emanates 

 from a point or a surface, as it is the degree of illumination of the 

 respective portions of the object glass which is measured. Errors 

 due to the unequal transmitting power of the different portions of the 

 lens, and also those due to the loss of light by reflection, &c, are 

 corrected by repeating the observations after the whole of the 

 apparatus has been reversed ; i.e., if the star is seen by reflection in 

 the first observation, it is to be viewed by direct vision in the second, 

 when the standard light will be viewed by reflection. 



The second method employed was a modification of Zollner's lamp, 

 the standard used being the author's pentane Argand burner, in which 

 air carburetted with pentane is employed as a combustible. This is 

 mounted on the eye-end of the telescope. Next to the burner is 

 placed a plate of ground blue glass, carefully adjusted to reduce the 

 colour of the gas flame, so that it is, as nearly as possible, comparable 

 with that of daylight. This glass is mounted permanently in front 

 of an aperture, 0'08 inch in diameter, in a brass plate. At a distance 

 of 2*4 inches from this is placed a second plate of brass, having an 

 aperture of 0*013 inch diameter. A lens, fixed at 2 inches from 

 this second plate, projects the image of the illuminated surface of the 

 blue glass by a reflecting prism, to the eye-piece of the telescope, 

 in which it is viewed as a circular disc of light, side by side with 

 that caused by the illuminated image of the telescope object glass. 

 The ground surface of the blue glass affords a slightly granulated 

 appearance, exactly imitating that of the object glass when a star 

 is viewed out of focus. . By arranging the position of the eye-piece 

 until the two images are exactly of the same size, the imitation of 

 a star is so striking that it is all but impossible, when the colours 

 are alike, to discern the difference. 



For the modification of the colour of the comparison light, two series 

 of coloured glasses are arranged in rotating diaphragms situated 



