120 
PHYSICS: C. BARUS 
The telescope T (relatively much enlarged in the diagram) is not 
attached to the block. It is to be used both as a simple telescope for 
the adjustment of the white slit images to horizontal and vertical coin- 
cidence, and as a direct vision spectroscope. The most convenient 
attachment for this purpose is the direct vision prism grating, G (film 
grating), just in front of the objective of T. Two perforated thin discs 
of brass are useful for this purpose, one disc being firmly attached (Kke 
the cap) to the objective, the other to the flat face of the grating with 
the prism outward. A swivel bolt, a, between the discs, thus allows 
the observer to throw out the grating and use the telescope. A stop 
arrests the motion of the grating when it is rotated about a, back again, 
for viewing the spectrum. This plan worked very well and the ellipses 
obtained were magnificent. It was now almost possible to control 
the micrometer M manually and all hurtful quiver is absent. The 
fiducial line to which centers of ellipses, etc., are to be returned is always 
the sodium doublet, present in sunlight or the arc and artificially sup- 
plied by an interposed burner in case of the Nernst lamp. The tele- 
scopic lens need not be more than 2 cm. wide and cross hairs are not 
needed. For measuring dispersion the Fraunhofer lines B, C, D, E, b, F, 
were used. 
4. Equations. The useful equation for present purposes are given 
in a preceding report"^ and the following cases only need be repeated 
here. If e is the thickness of glass plate of index of refraction co for the 
wave length X and if the equation /x = ^ + B/\^ where A and B are 
constants be taken as suflicient 
- 1 = AN/e - 2 B/\\ (1) 
where A7V is the displacement of the micrometer at the opaque mirror 
M or due to the insertion of the plate normally to the component 
beam in question. To determine /z, B must be known at least approxi- 
mately. It may be measured in the same adjustment, however, if 
two Fraunhofer lines are used fiducially. Let bN be the displacement 
of micrometer to pass the center of ellipses from wavelengths X to X^ 
Then if e' is the thickness of the half silvered plate H, and R the angle 
of refraction within it, 
aiV = 5(e + .'cosi? + 2(.+^)) (i-^), (2) 
If e = 0, 
6A.„=B(e'cosi?+^)(i-^) (3) 
