682 Proceedings of the Royal Society of Edinburgh. [Sess. 
The self-recording device consists of an arm CE, rigidly fixed to 
the pivot P, and therefore swinging round with the hoop, which has a 
spear F, whose point can be made to puncture the strip of paper GG, by 
pulling the trigger L on the handle. This strip is 13" long and 2J" high, 
and is kept in position by the wide rubber band seen at the top, on a cork- 
faced brass piece, bent into a circular arc having a radius of 6|", and which 
subtends an angle of 120° at its centre. This arrangement for holding the 
strip is satisfactory in practice, and makes the changing of it an easy 
matter. After use, the strip is a permanent record of the intensities of the 
light transmitted by the instrument at the moments when the observer 
pulled the trigger Z. [By marking off a suitable scale on a transparent 
strip of celluloid, it is possible to read off without any calculation opposite 
the punctures on each strip of paper the corresponding percentages of light 
transmitted.] In order to avoid the possibility that a second puncture 
might be made by the spear at a place already occupied by a first, the 
return backwards of the spear is made to produce each time a slight 
permanent elevation of its point, so that the successive punctures take 
place higher and higher up on the face of the strip of paper. The 
mechanism which effects this is as follows : The rear end of the spear is 
pivoted, and the point end is weighted and suspended from a cord which 
passes over a pulley above and is then led back to, and wound round, a 
small drum (not shown) in the part M. This drum is fixed on the same 
axle as a toothed wheel, which, by means of a ratchet attached to the spear, 
is made to revolve slightly at every retraction of the latter. Any lateral 
motion of the spear, which would of course impair the accuracy of its 
record, is prevented by means of a guide on each side of its point. 
In a former paper * I indicated the advantages of using self-recording 
apparatus in connection with ocular photometric work. Briefly recapitulated, 
these are : (1) saving of labour and of eye-strain, (2) elimination of possible 
errors of reading, (3) great saving of time, (4) the obtaining of a permanent 
record, and (5) elimination of personal bias. 
I am glad to take this opportunity of expressing my thanks to the 
Trustees of the Moray Fund for a grant to defray the cost of the apparatus, 
and also to the Carnegie Trust for a subsequent small grant which enabled 
me to carry out some necessary calibrations. 
Summary. 
This paper describes a new form of rotating photometric “sector/* 
capable of very accurate adjustment to any desired light-transmission value 
* Proc. Boy. Soc. Edin ., vol. xxix. p. 176, 1909. 
