238 Proceedings oj the Royal /Society 
mined that the length of the side should be 444 cubits, and accord- 
ingly measured out that distance ? 
“ Ex. 3. Again, if instead of taking Colonel Vyse’s measure of the 
base, we accept for that quantity the mean of half a dozen of the 
most trustworthy travellers’ statements, — for instance, Yyse’s, the 
French Academicians’, Caviglia’s, Wilkinson’s, Lane’s, and Davi- 
son’s, — we have for the length of the side in English feet a number 
which expresses in millimetres the mean height of the barometer 
at Upsala. But will any one maintain that the dimensions of the 
pyramid were intended by its builders to have any reference what- 
ever to that interesting constant ? 
“ Ex. 4. If we multiply together one-tenth of the side of the 
pyramid’s base, the length of the line joining the middle of the 
side and the apex (that is, the height of each one of the four 
isosceles triangles that compose the pyramid), and the modulus for 
the common logarithms, the result is 3420, the constant of lunar 
parallax, for which, in Burg’s tables, the value 3420"’96 is given. 
“Ex. 5. Lastly, if the side of the base (763-81) be divided by the 
hyperbolic logarithm of tv (the circumference to diameter), and 
that quotient again by the ratio of the force of gravity at London 
to the force of gravity at the pyramid (lat. = 30°), (1*00188), the 
result is 666 ! ” 
4. On the best Arrangement for producing a Pure Spectrum 
on a Screen. By J. Clerk Maxwell, M.A., F.R.SS. L. & E. 
In experiments on the spectrum, it is usual to employ a slit 
through which the light is admitted, a prism to analyse the light, 
and one or more lenses to bring the rays of each distinct kind to a 
distinct focus on the screen. The most perfect arrangement is 
that adopted by M. Kirchhoff, in which two achromatic lenses are 
used, one before and the other after the passage of the light through 
the prism, so that every pencil consists of parallel rays while pass- 
ing through the prism. 
But when the observer has not achromatic lenses at his command, 
or when, as in the case of the highly refrangible rays, or the rays 
of heat, he is restricted in the use of materials, it may still be use- 
