108 TRANSACTIONS OF THE [MAR. 9, 
adjustment of the lamp the current must be turned off, as the 
lamp cannot be handled with the current on without danger, 
possibly attended with fatal results. The current used for pro- 
jection should be of low tension, for with this the lamp can be 
handled without danger, and all adjustments may be made while 
the lamp is in operation. . . . The intensity of the are depends 
upon the current used and the size of the carbons. The inten- 
sity of the arc light has been variously estimated at from 400 to 
2,000 candle power. With a variable arc this will take place, 
but for purposes of projection this objectionable feature has been 
removed. With a proper position of the carbons these changes 
in the intensity of the light fall within narrow limits, and the 
fluctuation due to mechanical movements in the lamp is small, 
as the interval of feed is about two seconds. The intensity of 
the light when the carbons are arranged as described for projec- 
tion is from 1,200 to 1,500 candle power. 
The oxyhydrogen light under the best conditions will yield 
from 600 to 1,000 candle power. The character and color of 
the lime light are as different from those of the arc light as are 
its optical properties. The lime light has a yellow tinge and 
the volume is large, making it impossible to converge the rays 
to a focus. 
The are light has the advantage over all other methods of il- 
lumination ; its small cost per candle power, intensity, white- 
ness, and parallel rays make it especially well adapted for pur- 
poses of projection, as it admits a larger number of rays to pass 
through the optical centre of the condenser, and these rays can 
be brought to a perfect focus, thus producing a correct image 
with flatness of field, which is essential to produce sharp and 
well-defined pictures. 
This matter of increased intensity is of the utmost impor- 
tance, especially in large public halls where the pictures are en- 
larged many hundred times without regard to the diminished 
illumination produced by means of the oxyhydrogen light, 
which fails to bring out the details in the shadows, much to the 
disappointment of audience and producer of the slide. These 
considerations alone are enough to warrant its introduction for 
general use. When we consider that no skill is required in the 
operation of the lamp (other than to turn the switch and regu- 
late the resistance), [ feel that you will agree with me that the. 
are light, so many times denounced and said by some to be im- 
practicable for projection, has now become indispensable for a 
progressive community. 
At the close of the paper the Academy expressed its thanks to 
Dr. LAupy by a unanimous vote. 
