DR. W. HUGGINS ON THE PHOTOGRAPHIC SPECTRA OF STARS. 
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possible to keep the star in view upon the slit during the whole time the photograph 
is being taken, and to correct instantly by hand any small departure of the star’s 
image from its proper place upon the slit. 
I will now describe how the necessary breadth was given to the spectrum without 
the employment of a cylindrical lens. As the star’s image is not a point, its linear 
spectrum has a small breadth, but not more than about half the breadth which is 
necessary for the lines to be well seen. After the exposure had proceeded sufficiently 
to produce a linear spectrum, the image of the star was moved upon the slit in the 
direction of its length, through a space equal to about the apparent diameter of 
the star’s image. The exposure was then continued for a period of about the same 
length. In this way a photographic spectrum can be obtained of the breadth that is 
desired by the union of two or more linear spectra. 
The artificial light was thrown upon the silver plate by a small mirror fixed on the 
side of the telescope tube opposite to the end of the declination axis (cl, fig. 2). This 
axis is hollow and the light passes through it from a lamp suspended at the end. The 
precaution was taken of making this light pass through a plate of yellow glass. 
§ IV. Photography. 
At the early stages of these experiments I used wet collodion, but I soon found 
how great would be the advantages of using dry plates. Dry plates are not only more 
convenient for astronomical work, being always ready for use, but they possess the 
great superiority of not being liable to stains from draining and partial drying of 
the plates during the long exposures which are necessary even with the most sensitive 
plates. I then tried various forms of collodion emulsions, but finally gave up these in 
favour of gelatine plates, which can be made more sensitive. The development was 
sometimes by the ferrous oxalate process, at others by the ordinary pyrogallic method. 
Positives were taken from the original negatives by placing the negative plate upon 
a similar dry plate and exposing to a gas flame for two or three seconds. Some of the 
