The Position of the Bands in Sjpedra. By H. C. Sorhy. 273 
and the piece finished off true to less than Tp"oth of an inch. 
When once made, the only material ohjection is that since the 
quartz must he 1^ inch thick and there must be Nicol's prisms at 
each end, the entire length becomes about 3^ inches. For this 
reason it is the most convenient to place it in the position of the 
condenser under the stage of the microscope, and to make use of it 
with the binocular apparatus described in the paper alluded to 
above, care being taken to reflect the light directly through the axis, 
since otherwise there is a slight change in the position of the 
bands. By arranging a movable plano-convex lens over the upper 
Nicol's prism, as shown by Fig. 1, the light may be made equally 
bright in the two tubes of the microscope. The position of the 
bands seen in the spectrum of any object placed before the small 
reflecting prism can then be measured with great facility to some- 
thing like the millionth of a millimeter, since with well-marked 
bands consecutive measurements do not differ more than x^o^h of 
the half revolution of the circle, and when they do differ more the 
mean of several observations may be taken. 
As an example, I give the measurements in the case of the 
bands seen in the spectra of oxidized haemoglobin and of the same 
substance in which the oxygen is replaced by carbonic oxide. 
Measurements 
by Scale. 
Wave-lengths. 
2-19 
2-b8 
584 
547 
- 9 X 
- 8 X 
•6 = 
•4 = 
579 
544 
Carbonic oxide lisemoglobin . . | 
2-28 
2-96 
578 
543 
- 8 X 
- 6 X 
•5 = 
•5 = 
574 
540 
