ASTRONOMY. 207 



tiou of Mars tlirough a small hole l-lOOtli of a millimeter in diameter, 

 i. c, 1-lOOtli of the diameter of Mars, or about 40 miles, the diameter of 

 Mars iu the telescope being one millimeter. Much difficulty was expe- 

 rienced in making such a hole, and it was only after various attemi)ts 

 that Prof. Pickering succeeded iu boring suitable ho^es in a piece of 

 platinum foil gummed on a glass plate. 



The light of Mars seen through the smallest hole that could be made 

 was, after all, much greater than that of the satellite, which must there- 

 fore, if its albedo or reiiecting j)ower is about the same as that of JMars, 

 have a diameter much less than 40 miles. 



The following are the results of comparison with ]\Iars, as well as with 

 Saturn and certain faint stars near which the satellites passed : 

 Mars— 667,000 xPhobos in 1877; equiv. diam.=5.57 miles. 



Mars=879,000xDeimos in 1877; equiv. diam.=4,9+0.1 miles. 



Mars=550,000xDeimos in 1879 ; equiv. diam.=6.1±0.1 miles. 



Mars=461,000xDeimos foil. Mars; equiv. diam.=6.7 miles. 



Mars=673,000xDeimos prec. Mars ; equiv. diam.=5.o miles. 



The equivalent diameter of the satellite is calculated on the assump- 

 tion that its albedo is the same as that of Mars. 



It appears, then, from Prof. PickerinGt's results that Deimos was ap- 

 parently much brighter in 1879 than in 1877, and that it was brighter by 

 nearly half a magnitude on the following side than on the preceding 

 side of its orbit. In support of this Prof. Pickering points out that 

 there is a large preponderance of micrometer observations of Deimos on 

 the following side, showing that the satellite was more easily seen iu 

 that part of its orbit. The micrometer measures of Pliobos were nearly 

 equally distributed. No photometric measures were made of this satel- 

 lite in 1879. The general result of all the observations is that Deimos 

 is about 14.5 magnitudes (on the photometric scale) fainter than Mars, 

 and about 0.3 magnitude fainter than Phobos, the equivalent diameters 

 of the two satellites being about 7 and 6 miles respectively, correspond- 

 ing to 0".010 and 0".009 at mean distance. That is, the satellites are 

 fainter than an area of the surface of Mars having a diameter of 0".01 

 at distance unity, or 1-lOOOth part of the diameter of Mars. isTotwith- 

 standing the minuteness of the i)oints of light compared, a difference 

 iu color was detected between the outer satellite and Mars, Deimos ap- 

 pearing to be blue, w^hilst the artificial star formed by Mars was noted 

 as red. 



The observations of the satellites of Jupiter in Chapter VIII are of 

 special interest, from the comparison of equivalent diameters (photo- 

 metrically inferred) and the diameters as actually measured. The fol- 

 lowing are the results : 



Satellite - I. II. III. IV. 



Light ratio - - - 1786 2032 1271 3597 



Equivalent diameter 0".873 0".818 1".035 0".615 



Measured diameter 1".081 0".910 l".r,37 1''.282 



Relative albedo 0.652 0.809 0.453 0.230 



