Spectrophotometer of the Hilfner lype. 287 
If a gives the position of the nicol, the fraction of the 
intensity of the incident beam transmitted by the solution 
is given by 
T _ j tan 2 a, 
i " 1 °tan2 51 0, 
££If c denote the concentration of the solution in gram- 
molecules per litre, and d the thickness of solution passed 
through, then A the " molecular extinction coefficient " is 
■defined by 
i=i io- a «*. 
The following table gives the results : — 
a. 
Time of 
exposure. 
X. 
A. 
A from the 
tables. 
49-S 
10 mins. 
4750 
0053 
0-009 
474 
5640 
0187 
0-29 
45-0 
jj 
5710 
0-322 
037 
41-6 
M 
5850 
0-514 
0-57 
381 
,, 
5980 
0-714 
0-83 
300 
15 mins. 
6130 
1-22 
1-24 
231 
20 mins. 
6200 
1-69 
1-50 
149 
5> 
6410 
2-45 
2-42 
c = , 5424' gm.-rnolecules/litre. 
d=l cm. 
The width of the D line on the plates is 0*6 mm. 
One exposure gives A for only one wave-length, but if the 
absorption had more than one maximum the exposure might 
give A for several values of X. The values of A taken from 
the tables are of course more accurate. I attempted to 
determine the wave-length by putting a piece of didymium 
glass before the slit, but this method was not so accurate as 
the one used. 
The cost of the instrument has been borne by the Carnegie 
Trust for the Universities of Scotland. 
X2 
