1897] MICROSCOPICAL JOURNAL. 349 



feet. Stability and freedom from vibration are very 

 easily obtained with the small and short apparatus 

 The difference in cost is enormous. In several re- 

 spects the phofcomicrographic arrangement has advan- 

 tages over the great Lick photographic instrument. 



If, however, Ave turn to the matters of liglit and 

 separating power, the very great superiority of the 

 Lick objective is seen. The results given in the fol- 

 lowing tabular comparison are only approximately 

 accurate. The loss light suffers by absorption as it 

 passes through glass and by reflection at incident sur- 

 faces, is not taken into account; — the Lick objective con- 

 sisting of three thick lenses and the photomicro- 

 graph ic arrangement having more than twice as many, 

 but comparatively very thin, lenses and the mirror's 

 reflecting surface: 



Lick Larger Smaller 



Objective. Portrait Leus. Portrait Lens. 



Diameter of objertive 33 in. 3.75 in. 2 iu. 



Foens of objective 550 in. 15 in. 8 in. 



Focus divided by diameter 16.66 4 4 



Relative value oflight in fir.st 



iniao;e 1 16 16 



Size of li ret image 5.1 iu .1395 iu. .0744 in. 



Total equivalent focus, 550 in- 

 ches, divided by diameter 16.66 147 275 



Relative value of light iu final 



image 1 ^] 277 



Time of exposure, eclipse of sun i i i 



(about) . i^'on «ec. ^osec. .sec. 



Separating power 1 ^^ ^^1,,^ 



Other things being equal, separating power varies 

 with the aperture or diameter of the objective. If the 

 lick objective, having an aperture of thirty-three inches, 

 could barely show a certain double star as two distinct 

 stars, it would be impossible for any objective having an 

 aperture of four or two inches to show such a double 

 star as two distinct stars. A star apparently single when 

 seen through any objective having an aperture of two 



