542 
CAPTAIN W. DB W. ABNEY AND MAJOR-GENERAL E. R. BESTING 
aperture being rendered light-tight by a batten. The screen is black except one 
circular patch which can be altered at pleasure in colour or size, but which in the 
experiments now to be described was white and § inch in diameter. 
When using this instrument the beam to be extinguished was directed through 
the tube T and diaphragm D on to the ground glass by which it was diffused. A 
portion of the diffused beam was reflected by the mirror M to the white patch on the 
screen at S. By altering the diaphragm D the amount of light falling on .S can be 
varied at pleasure, and it can be still further regulated by putting the rotating sectors 
in the path of the incident beam outside T. 
Fig. 39. 
Screen for measuring illumination. 
The point of extinction was observed as follows. The slits of the collimator and of 
the slide were closed to convenient widths, and the light was subsequently diminished 
by inserting diaphragms. Two methods of extinction were tried, (1) The slit tra¬ 
versing the spectrum was moved until the ray was found which was just 
extinguished with each diaphragm; and (2) after placing the slit in fixed positions 
in the spectrum at a known ray the light was diminished by the rotating sectors as 
well as by the diaphragms. The latter is evidently the more convenient plan, but 
both were fully tried in order to determine whether the method of reducing the light 
by the rotating sectors could be relied on in experiments of this nature. The agree¬ 
ment between the results obtained, which was as close as could be expected in such 
experiments, convinced us of the trustworthiness of the latter method. 
A means had to be devised by which a beam of sufficient intensity to be easily 
measured could be reduced to the point of extinction, and the proportion in which it 
had been reduced ascertained. The slit slide was taken away from the spectrum, and 
the intensity of the re-combined beam determined in terms of the light of the amyl- 
acetate lamp as follows. A card (fig. 39) was pierced with a square aperture B, as 
shown, and a piece of Saxe paper pasted over the whole. A black paper mask was 
then applied, so as to leave B and an equal area A visible. From one side the paper 
