1910 - 11 .] 
On Photometric Paddle-Wheels. 
681 
to fix it to the motor axle, and also a brass rim T W thick and J" wide 
for the attachment of the vanes. The latter, nine in number, were cut 
out of sheet brass by the aid of a template, and are fixed in slotted 
studs screwed into the brass rim. A simple optical method was employed, 
by means of which each vane in turn was accurately set at the desired 
angle of 60 ° with the axle ; and after each one was adjusted, it was finally 
soldered so as to guard against any possibility of its working loose and 
changing its position. 
Fig. 17 . 
The whole apparatus is enclosed from light by means of two sheet 
metal covers, one of which is shown in position in fig. 16 , the beam of light 
passes through the (very short) tube Q, across the top of the wheel, and 
out through a similar tube on the other side. Velvet bags attached by 
rubber bands to these tubes make a light-tight connection with correspond- 
ing short tubes on the neighbouring parts of the spectro-photometer. The 
instrument is provided with a handle B, fixed to the pivot P, on which the 
observer keeps his hand for the purpose of altering the azimuth of the 
hoop, and thus changing the intensity of the beam of light. 
