1902-3.] Mr J. R. Milne on the Spectrophotometer. 
499 
spectrum can be easily mapped by Murphy’s method.* In this 
method the spectrum is divided into a large number of narrow 
strips. Starting from one of these, the intensity of one of its two 
neighbouring strips is measured in terms of the first as a standard. 
This other is then in its turn used as a standard for measuring the 
intensity of the next in succession, and so on, till the whole 
spectrum has been covered. Figure 2 shows the two spectra 
formed by my apparatus, arranged for Murphy’s method. These 
two spectra are formed from one and the same light, and are 
therefore identical in all respects. One has been displaced, how- 
ever, slightly to the right, the other a corresponding amount to 
the left. This displacement is accomplished by moving the cor- 
responding half-lenses to right and left respectively, by means of 
the adjusting screws. The relative intensity of strip A' with 
regard to B may now be measured by the spectrophotometer’s 
graduated apparatus for the purpose, which cuts down one beam 
until it is of the same brightness as the other. But A' is identical 
in every respect with A, hence B has been measured relatively to 
A'. In this way the whole spectrum may be measured step by 
step. 
So far Only a rough and simple apparatus has been made, which 
has been brought here and set up, so that it may be inspected by 
anyone interested. 
This apparatus consists of three separate parts, each of which is 
mounted on its own stand. These stands are placed one behind 
the other, and are arranged in line with the telescope tube of the 
spectrometer. First comes a screen having a wide horizontal slit, 
whose width can be altered at pleasure. This screen is so placed 
that the two spectra formed by the objective lens of the spectro- 
meter telescope are situated in its plane at the height of the slit. 
This screen is necessary to ensure sharpness in the two edges of 
the spectra, which are afterwards to be brought in contact with 
each other. The next part consists of a vertical plate, in whose 
aperture are situated the two half-lenses ; the narrow gap between 
them being horizontal (the slit of the spectrometer is vertical). 
These half -lenses can be adjusted with relation to each other by 
means of two slow-motion screws, one giving an up-and-down 
* See Astrophysical Journal , xi. 220, 1900. 
