ASTRONOMY. 207 



tion of Mars througli a small bole 1-lOOtli of a millimeter iu diameter, 

 i. e., 1-lOOtli of the diameter of IMars, or about 40 miles, the diameter of 

 Mars in the telescope beiug- one millimeter. IMncli difficulty was expe- 

 rienced iu making such a hole, and it was only after various attempts 

 that Prof. Pickering- succeeded in boring suitable holes in a piece of 

 j)latinum 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 reflecting power is about the same as that of Mars, 

 have a diameter much less than 40 miles. 



The following are the results of comparison with Mars, as well as with 

 Saturn and certain faint stars near which the satellites passed : 

 Mars=GG7,000xPhobos in 1877; equiv. diam.=o.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.=C.7 miles. 



Mars=G73,000xDeimos i)rec. Mars ; equiv. diam.=5.5 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. Pickering'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 in 

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

 equally distributed. Ko 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. Notwith- 

 standing the minuteness of the points of light compared, a difference 

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

 pearing to be blue, whilst the artificial star formed by Mars was not^d 

 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 arc the results : 



Satellite I. 11. III. IV. 



Light ratio 1786 2032 1271 3597 



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



Measured diameter 1".0S1 0".910 l".r)37 1".282 



Relative albedo 0.652 0.809 0.453 0.230 



