This arrangement leaves untouched the old difficulty in photometry, that arising from the different colors of different lights.  I propose to obviate that difficulty in the following manner.  As is well known, gold transmits the green rays, silver the blue rays, and so on; therefore, a cell faced with gold will be acted upon by the green rays, one faced with silver by the blue rays, etc.  Now, if we construct three cells (or any other number), so faced that the three, collectively, will be acted upon by all the colors, and arrange them around the light to be tested, at equal distances therefrom, each cell will produce a current corresponding to the colored rays suited to it, and all together will produce a current corresponding to all the rays emitted by the light, no matter what the proportions of the different colors may be.  The three currents may act upon the same index, but each should have its own coil, not only for the sake of being able to join or to isolate their influences upon the index, but also to avoid the resistances of the other cells.  If a solid transparent conductor of electricity could be found which could be thick enough for practical use and yet would transmit all the rays perfectly, i.e., transmit white light unchanged, that would be still better.  I have not yet found a satisfactory conductor of that kind, but I think the plan stated will answer the same purpose.  This portion of my system I have not practically tested, but it appears to me to give good promise of removing the color stumbling-block, which has so long defied all efforts to remove it, and I therefore offer it for your consideration.

Photo-electric regulator.—­My regulator consists of a current-generating cell arranged in front of a light, say an electric lamp, whose light represents the varying strength of the current which supports it.  The current produced in the cell by this light flows through an electro-magnetic apparatus by means of which mechanical movement is produced, and this motion is utilized for changing resistances, actuating a valve, rotating brushes, moving switches, levers, or other devices.  This has been constructed on a small scale, and operates well, and I think it is destined to be largely used, as a most sensitive, simple, and perfect regulator for currents, lights, dynamos, motors, etc., etc., whether large or small.

In conclusion, I would say that the investigation of the physical properties of selenium still offers a rare opportunity for making very important discoveries.  But candor compels me to add that whoever undertakes the work will find it neither an easy nor a short one.  My own experience would enable me to describe to you scores of curious experiments and still more curious and suggestive results, but lack of time prevents my giving more than this very incomplete outline of my discoveries.

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ELECTRICITY APPLIED TO THE MANUFACTURE OF VARNISH.