108 TRANSACTIONS OF THE [ilAR. 9, 



adjustment of the lamp the current must be turned off, as the 

 lamp cannot be handled witli 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 arc 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 oxyhydrogeu 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 arc 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 lise. 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), t feel that you will agree with me that the 

 arc 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. Laudy by a unanimous vote. 



