PHOTOGRAPHY IN THE SERVICE OF ASTRONOMY. 477 



that the auginentation does uot follow a wimple law. Under the mi- 

 croscope* these round black spots are resolved into a multitude of 

 black points, very crowded at the center, for the stars of the first ten 

 magnitudes, more and more thinly distributed for the fainter stars, 

 down to the doubtful traces which mark the extreme limit of chemical 

 sensitiveness. At present this limit is much further removed than 

 that of the penetration of the eye armed with a telescoi)e. The dotted 

 character of the images proceeds evidently from the action of light 

 upon the molecules of the salt of silver incorporated in the sensiti\'e 

 film. These photographic stars resemble thus clusters of stars, or 

 iiebuhe more or less resolvable. 



This aspect is so characteristic that there is little risk of confounding 

 very small stars with accidental spots, as was feared at first, and it fol- 

 lows that a duplication of exposures may often be dispensed with. 

 Messrs. Henry, to avoid all confusion, have confined themselves to re- 

 peating three times the ex])osures on the same plate, by displacing the 

 telescope each time in such a way as to form with each star a small 

 equilateral triangle of 3 to k seconds on a side. This triangular appear- 

 ance is uot at all perceptible excei)t with a lens; the paper prints give 

 images which appear perfectly round. A subsidiary advantage of this 

 mode of operating is that it thus becomes possible to remove iarther yet 

 the limit of visibility ot the stars ; thus may be established very easily 

 in this manner the presence of an unknown planet, whose proper mo- 

 tion would deform the microscoi»ic triangle. But it is clear that the 

 trii)le exposure involves a great loss of time. The congress has pre- 

 ferred for the execution of the chart of the sky, as we have seen, two 

 parallel and independent series of plates. 



The i)lates will not acquaint us with the absolute positions of the stars; 

 they will only permit us to determine their relative situations. It is also 

 necessary, in order to obtain these with the desired precision, to i)ro- 

 vide a system of standards. These stamlards will be i)rocured by the 

 re-production of the reticules Mr. Vogel prepares for this object, and 

 which are traced with a steel point on plates of silvered glass; placed 

 upon the sensitive plate, the reticule leaves there a latent image, which 

 develo[ied later appears under the iorm of a system of very definite lines 

 of reference. These standard reticules aie not only of great assistance 

 in micrometrical measures of stellar positions, but they will serve to 

 control the deformation of the gelatine film. We know that for collo- 

 dion the shrinkage and the deformation of the image which it entails 

 can attain an amount very sensible; it is no greater with gelatine, 

 which adheres very strongly to the glass. This is less according to the 

 micrometric comparisons of an original reticule and several photo- 

 graphic copies, which Mr. Scheiner has recently undertaken ; but we 

 cannot answer for the invariability of the plate in each particular case 



* In the absence of a lens a simple card picicod with a small hole which is held be- 

 fore the eye may be used to examine these images ; it is a primiiive lens. 



