656 
Proceedings of the Royal Society of Edinburgh. [Sess. 
XLYI. — On Photometric Paddle-Wheels. By James Robert 
Milne, D.Sc. 
(Read March 21, 1910. MS. received August 2, 1911.) 
CONTENTS. 
PAGE 
§ 1. Introduction 656 
§ 2. Different Types of Paddle-Wheel 658 
§ 3. The Relation between the Angle of Azimuth of the Axle of the 
Paddle-Wheel and the Corresponding Reduction of the Light 
Intensity ... 663 
§ 4. The Condition necessary to ensure that all the Rays of the Beam 
of Light shall have sensibly the same Intensity after their 
Transmission by the Paddle-Wheel 667 
§ 5. The “Contact Condition” ... 676 
§ 6. On what is necessary to satisfy both the foregoing Conditions . 679 
§ 7. Description of an actual Paddle-Wheel Instrument, and of its Self- 
recording Device for Registering the Readings . . . . 680 
§ 1. Introduction. 
The purpose of the following paper is to describe a modification of the well- 
known rotating sector, so extensively used in photometric investigations. 
What led the writer to devise this new form w r as the need for a sector 
apparatus, or its equivalent, which would allow of varying the intensity of a 
beam of light without stopping the rotation of the sector, and which would 
at the same time give accurate readings. In the ordinary form of rotating 
sector, consisting of two sectored discs fixed to a common axle by a thumb- 
screw, there is of course no possibility of altering the angle of the sectors 
while the apparatus is in motion. This drawback is partially overcome in 
the “ stepped sector ” now so much employed in photographic researches, 
and in the ingenious modification of the stepped sector due to Mr J. de 
Graft Hunter : * but both these forms only permit of the use of a limited 
number of fixed light-reduction ratios. Neither of these objections applies 
to Sir W. de W. Abney’s sector, with which the light-reduction ratio 
can be continuously modified, and its value also read at any time 
without stopping the rotation of the sectors. It is evident, however, 
that great accuracy cannot be looked for in any form of mechanism 
which has to perform the duty of imparting small relative displacements 
to two parts each of which is moving with a high velocity. The principle 
* Proc. Roy. Soc., Series A, vol. lxxxii. p. 307, 1909. 
