836 Proceedings of Royal Society of Edinburgh. [sess. 
The galvanometer employed had a resistance of 8498 ohms. 
The scale was 15 feet from the mirror, and the source of illumina- 
tion was a Nernst lamp. The non-polarizable electrodes were of 
the trough form, first used by Du Bois Reymond.* We con- 
structed a dark box, of which we give a drawing in outline. The 
box is 29 cm. in length, 10*5 cm. in breadth, and 11 cm. in height. 
On the bottom is a groove, having thick edges, which is exactly the 
width of the troughs, so that these can he kept securely in position, 
while they can be slid longitudinally (that is, approximated or sepa- 
Arrangement for experiments with Radium on Frog’s eye. A.B. Non-polariz- 
able electrodes. O. Holder for radium. D. Dotted line shows an out- 
line of the lead plate, which can be interposed between radium and eye. 
The dotted outline shows the dark box, with the lid open, so as to admit 
light, when radium was not used. 
rated) as required. In the middle of the upper surface of the box, 
corresponding to the lid, there is a little shutter for admitting or 
excluding light. The box is provided with binding screws, by which 
the troughs may be connected with it and with the galvanometer. 
On one side of the box, just in the middle, and at a sufficient height, 
we cut a rectangular hole through which we passed an accurately 
fitting piece of lead 85 mm. in length, 20 mm. broad, and 3 5 mm. 
in thickness. The end of the lead inside the box ended in a 
T-shaped portion, the T being 38 mm. in length. This piece of 
* These I used because I have been accustomed to them for thirty-five 
years. — J. G. M‘K. 
