16 GS. Sellack—Photography of Southern Star-clusters. 
The adjustment of the lens was to be done by continued 
hotographing of stars, as the image is not sharp for mean visi- 
le rays, and even observation through a blue glass did not 
show it distinctly enough. The lens had to be taken out of 
the telescope for each correction, which was done many a hun- 
dred times. The image of the star was photographed, not only — 
in its true focus, but at different distances before it, so that the 
section of the converging light was obtained; the shape and 
arrangement of light in these sections allowed a better judg- 
ment of the state of the different parts of the lens. As the ob- 
ject was not the adjustment of parts of a simple lens, but of the 
diverging lens of a system, it is obvious that mistakes in cor- 
recting the screws were easily made, Displacement of the two 
poe of the lens produces double images in the manner of the 
eliometer, except that in this case the concave lens alone 
makes the effect; tilting of the pieces about the edge of the 
erack has a similar effect. e surface of the crack being very 
irregular, it was difficult to fit the pieces exactly together. 
tude, as of several stars in the Pleiades. The image of the 
broken lens does not seem to have the same concentrated sharp- 
ness; only slow moving stars, fainter than third magnitude, 
leave a trail. However, with exposures of eight minutes stars of 
the ninth magnitude, of white color, give a photographic impres- 
sion; this result does not seem to have been surpassed formerly. 
The greatest difficulty in stellar photography is to make the 
image on the plate stationary during a long exposure. The 
steadiness is absolutely necessary for the production of circular 
images; the images must be circular, because in elliptically 
lengthened images the eye cannot fix the center with the sharp- 
ness required for the measurements. Employing even the most 
perfect clockwork, the steadiness of the image is affected by the 
effect of the atmospheric refraction, by the variations in the 
refraction produced by disturbances in the atmosphere, and by 
the increase of refraction dependent on the zenith-distance. 
The photographic nage of stars is circularly spread by pro- 
longation of exposure; this is principally the effect of the scin- 
tillating motion of the image, not of want of definition, as its 
amount depends on the state of the atmosphere. Bond has 
ee lm 
