181 
1908-9.] A Special Form of Photographic Camera. 
The advantages of this method of making photographic records of the 
readings of an instrument are — 
(1) The saving of labour and strain to the eye of the observer, which is 
left in a better condition for its principal work. 
(2) The elimination of the possibility of errors of reading on the part 
of the observer. 
(3) The great saving of time which is effected. 
(4) The elimination of all personal bias due to a knowledge of the 
results that are being obtained. 
(5) The securing of a permanent record of the readings. 
(6) The entire separation of the recording apparatus from the polari- 
meter or other instrument, whose mechanism is not added to or interfered 
with in any way. 
I have to express my great obligation to the Carnegie Trustees for 
granting me the funds necessary to defray the expenses of the construction 
of the apparatus. 
Department op Physics, 
Edinburgh University. 
(Issued separately April 17, 1909.) 
