February 1, 1887.] 



♦ KNOWLEDGE ♦ 



77 



Among the star clusters of which we have obtained satis- 

 factory photographs, we present here the cluster in Gemini 

 (Messier 35),* that magnificent cluster, 19' in diameter, 

 composed of stars from the eighth to the fourteenth mag- 

 nitudes, whose arrangement has struck all observers. We 



Fig. 2.— Photograph of Vega axd irs Compasioss. 



find here an astonishing stellar wealth which charting can 

 only reproduce laboriously and incompletely ; but photography 

 reproduces it faithfully (fig. 1). 



Let us note also in the same class of work, the group of 

 the small companions of Vega (fig. 2), and the group of the 

 fine quadruple star e Lyrse (fig. 3). The magnitudes in the 



instead of two neat pairs. But as will presently be shown, 

 these double stars have been photogi-aphed separately with a 

 very short exposure (a quarter of a second). 



Among the double stars which we have photographed and 

 measui-ed (in 1886), we may cite : — 



Magnitudes. 



61 Cygni 5' 5 and 6 



f Ursie Majoris 2 5 and 4 



IT Boot is 5 and 6 



y Virgiuis 3-0 and 3-2 



44 Bootis 5'3 and 6 



a Herculis 3o and .5'5 



p Herculis 40 and 51 



5 Serpentis 4 and 5 



e' LyrsB 6 and 7 



€^ L3-ra3 ou and 6 



These direct photographs enable us to measure on the plate 

 itself the position-angle of the components as well as the 



Fi(j. 4. — Path of the Double Star Mizae ox the 



Photographic Plate. 



Exposure such as to show the smaller star, and therefore to present 



the larger as a disc. 



distance. To determine the angle of position, we fii-st, as 

 in direct observation, allow the pair to advance in the 

 direction of the diurnal motion, photographing itself a certain 

 number of times along its line of motion. AVe thus obtain 

 a result such as is shown in fig. 4. This line of motion is, 

 of course, an east-and-west line, self-recorded with precision. 

 Consequently, starting from this line, we measure the posi- 

 tion angle on the plate itself. 



In the case cited the star photographed is Mizar ; the 

 north (0°) is below, the east (90°) to the right, and the 

 angle of position is 1-19°-1. 



Fig. 



Fig. 3.- 



-Photogbaph op the Group Epsilon 

 Ltrjv. 



former group are from the 1st to the 16th ; for the latter 

 from the 5th to the 15th. In the latter group, irradiation 

 resulting from the length of the exposure necessary to show 

 the very faint stars has combined the two discs of each paij. 

 into one, in such sort that we have two monstrous discg 



• It lies within the limits of the Milky Way, just north of the 

 ecliptic and east of the solstitial colure. 



-Paths of the Double Stars k Bootis, 58 Corvi, asd y Viegixis. 

 Exposures varying according to the conditions. 



[Fig. 5 shows the pairs k Bootis, 58 Corvi, and y Virginis. 

 It will be noticed that although t: Bootis is only a fourth- 

 magnitude star, and 58 Corvi below naked-eye vision, while 

 y Virginis is a third-magnitude star, these dififerences are 

 not recorded in the photographs, the exposure necessary for 

 obtaining photographic records being ditlerent in these 

 diflPerent cases. The double stars which lend themselves 

 best to the photographic method are those whose components 

 are nearly equal.] 



Messrs. Henry describe and picture an instrument for 

 magnifying and measuring the photographs of double stars, 

 " a sort of stellar microscope, which gives the distances of 

 double stars to the hundredth part of a second, by a process 

 akin to micrometric work. Astronomers and men of science 



