46 
Proceedings of the Royal Society of Edinburgh. [Sess. 
IV. — On the Illuminating Power of Groups of Pin-hole Burners. 
By R. G. Harris, Physical Laboratory, Edinburgh University. 
Communicated by Professor MacGregor. 
(MS. received August 19, 1909. Read July 12, 1909.) 
The object of the present series of experiments is to find the variation of 
the illuminating power of symmetrical groups of pin-hole burners with the 
distance of adjacent burners. 
The burners were fitted so as to slide on a frame composed of two pieces 
of a metre-rod fastened together to form a cross, as shown in figs. 1 and 2. 
The stems of the burners were circular in section and cylindrical in bore, 
the external and internal diameters being 8 mm. and less than 1 mm. 
respectively. Their bases were bevelled in such a manner that the burners 
could be fixed so close as to touch each other ; thus their centres could be 
placed at any distance from the centre of the cross from 4 mm. to 10 cm. 
The standard of illumination with which they were compared was a similar 
pin-hole burner. The photometer was of the Bunsen type, but instead of 
the grease-spot an arrangement described in Stine’s Photometrical Measure- 
ments, p. 69 (1900), was adopted. A square hole with sharply defined edges 
was cut in a piece of cardboard, and a thin piece of paper, of uniform texture, 
was firmly attached to each side of it. The screen was placed in a wooden 
fork, fixed to a large wooden block ; it was securely held by the fork without 
being permanently fixed to it, and could therefore be reversed as desired. The 
screen was viewed by mirrors placed one on each side of it, and inclined at 60° 
to it. For convenience in observing, a framework of black cloth was added. 
All the burners were connected to water manometers made of bent glass 
tubes. The manometers were used merely to see that the pressure was 
constant during any one set of observations. The pressure was always 
between 5 2 and 7*3 cm. of water above the atmospheric pressure. 
In practice the standard burner and the frame carrying the group of 
burners were fixed, the distance between the centre of the standard burner 
and the centre of the frame being 150 cm. This was measured by means 
of metre-rods screwed to the table. The photometer block was slid by hand 
along these metre-rods until equal contrasts were observed in the mirrors ; 
for the spot of light never disappeared simultaneously on both sides of the 
screen. It was difficult to judge this exactly, therefore ten observations 
were made in each case, the photometer screen reversed, and other ten 
