206 ASTRONOMY. 



personal equation or variations due to tlie observer, it is very desirable 

 that the objects shall resemble each other as mnch as possible. This 

 result is secured by bringing the images to be compared close together, 

 so that they are both viewed with the same aperture and magnifying 

 l^ower, and the light of both is equally distorted by passing through the 

 same lenses and x>risms. A detailed description is given of the many 

 photometers employed.* 



These photometers could only be used for comparing objects very 

 near together, such as double stars or satellites. For somewhat greater 

 intervals two achromatic prisms of small angle were placed in front of 

 a telescope, covering the central part of the object-glass. Two images 

 of any object would thus be formed, separated by an interval dei^end- 

 ent on the angle of the prisms, and on their relative positions. 



All the i^hotometers described are open to the objection that the loss 

 of light is very great, from CO to 80 i^er cent. This was especially felt 

 during the observations of the satellites of Mars, and led to the inven- 

 tion of another class of photometers. The image of some bright star, 

 assumed as standard, passes outside the telescope, and is reflected into 

 the field, after having been reduced by some known amount, until it 

 equals the faint object to be measured. 



The instruments having been described at length in Chapter I, the 

 second chapter contains the journal of the observations, while the dis- 

 cussions are given in Chapters III and IV. Conjunctions of planets, 

 already alluded to above, afforded a good opportunity for comparing 

 their relative brightness. The results are : 



Albedo of Saturn Albedo of Jupiter ^ or' r n no 



All — 1 i- A> = 4.62±0.07 -r-p, — ^ ^ ^r = 0.864-0.02. 



Albedo of Mars Albedo of Venus 



A series of i)hotometric measures of all the more conspicuous double 

 stars is discussed in Chapter TV. 



Part II of vol. XI of the Annals of Harvard College Observatory con- 

 tains the determinations of the brightness of various satellites. In these 

 researches Prof. Pickering had great difficulties to overcome on account 

 of the extreme faiutness of some of the objects whose light was to be 

 measured; the results, however, are of special interest, as aflbrding 

 some idea of the real size of these minute members of the solar system. 

 Prof. PiCKERiNa's attention was first directed to the newly discovered 

 satellites of Mars, and he devised various new forms of photometer for 

 the examination of these faint objects. These photometers are all de- 

 scribed in Part I, and the first two chapters of Part II (Chaps. V and 

 VI) are devoted to determinations of their constants, by various meth- 

 ods. The problem was one of great difficulty, involving the measure- 

 ment of a photometric interval as great as that between the Sun and 

 the Moon, the planet Mars being several hundred thousand times as 

 bright as his satellites. As a direct comparison was impossible, the 

 light of the satellite was compared with that received from a small por- 



*An abstract of this description is given in Nature, XXI, p 23. 



