WHILE engaged on work in connection with the discharge of electrification by ultra-violet light, we have come across a fact which it may be convenient to state by itself, viz. that the spectrum of an arc can be made visible over the greater part of its immense range of action on electrified metals, by receiving it upon a screen of the double fluoride of uranium and ammonium, such as is frequently used for displaying X-rays. The arc-light must, of course, be passed through a quartz train, and a long arc is best, especially an arc containing aluminium; but under these circumstances, whereas the ordinary visible spectrum may be three-quarters of an inch broad, a breadth which may be doubled or trebled by the use of ordinary fluorescent substances, the spectrum received upon a uranium screen is five inches broad, and is full of bright lines.