176 
Proceedings of the Royal Society of Edinburgh. [Sess. 
XI. — A Special Form of Photographic Camera for Recording the 
Readings of the Scales of Scientific Instruments. By James 
Robert Milne, D.Sc. 
(MS. received February 23, 1909. Read December 21, 1908.) 
Some years ago, when employed in work that involved the use of a 
polarimeter, there was brought forcibly to my notice the great loss of 
time which the frequent reading of the Nicol dial necessitates. To avoid 
this, I had a special form of camera made for the purpose of recording 
the readings automatically. This method proved successful from the first, 
but it is only latety that the mechanism of the camera attained its final 
form ; for it has required several years of experiment and alteration to 
evolve a form of camera which should be wholly automatic and yet 
reasonably simple. It will be readily understood that the function of the 
camera is to make a series of photographs of the scale-and-vernier of the 
polarimeter or other instrument, which, on subsequent examination, will 
show the relative position of the scale-and-vernier at the time of each 
exposure. 
When in use, the camera is so placed that its lens is focussed on the 
scale-and-vernier, and the latter is lit up by an incandescent gas burner 
with a piece of ground-glass interposed to diffuse the light evenly, or by 
an incandescent electric lamp having a “ frosted ” bulb. The scale-and- 
vernier must be so arranged that it is the scale, and not the vernier, which 
moves. The camera can be placed at any reasonable distance from the 
polarimeter according to the focal length of the lens employed, and of 
course it is not attached to the polarimeter in any way. Nor need 
the camera be placed directly at right angles with the scale-and-vernier 
to be photographed, as I find that a mirror can be used to deflect the rays 
of light, which allows of the camera’s being placed on one side. The mirror 
need not be silvered on the front surface, as the reflection from the glass 
is not sufficiently bright to give rise to a double image on the negative, 
hence a piece of ordinary plate mirror-glass answers very well. If it be 
desired to keep the room dark, it is not a difficult matter to arrange some 
form of covering for the path of the light on its way to the camera. 
The exposure required with the ordinary silvered brass scale is only a 
fraction of a second. 
A single plate is sufficient for many records, as only the vernier and the 
