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APPARA TUS FOR SCIENTIFIC DEMONSTRA TION 163 with the small-sized Siemens' differential lamp, and also with the Giilcher, which is very steady if a proper resistance coil be interposed in the circuit. Others also doubtless give good results. But the most important point in an optical lamp is the concentration of the radiant into one luminous point. All the usual lamps fail in that particular; and in perfecting the arrangements of the electric projecting microscope, especially, I was continually baffled by the confusion caused by the two luminous points or poles, which was only partially remedied by setting the positive carbon behind the negative. For some time, pointing this out, I endeavoured in vain to induce various makers to construct an optical lamp with carbons inclined at an angle of 30° or 40°, after the manner well known in the hand-regulated ' projector ' lamps often used at sea. Being finally advised by Professor S. P. Thompson to adopt for other reasons the Brockie-Pell lamp for microscopic work ; and finding upon inquiry that Mr. Brockie had already constructed tot Mr. Phillips a simpler arc lamp for lantern ?jse, adjusting the arc by the potential of the current, in which the negative carbon worked up by a kind of candle-spring to a fixed stop, so as to ' keep in focus,' while the arc could be readily adjusted to any current within reasonable limits, I urged upon him very strongly to further perfect it for some microscopes then constructing at Messrs. Newton's, by giving the carbons the slant just alluded to. He at once undertook to do this (my sole suggestion in the matter), and the final result of the additional alteration was a lamp whose external appearance can be seen in the representation of the electric microscope on p. 209. This, as an optical radiant, is far the best I am acquainted with ; and up to this date is, I believe, still the only focus-keeping regulator-lamp whose carbons occupy the proper inclined position. It is extremely port- able, steady upon its base, very simple hi mechanism, and its cost is only fifteen pounds. It is not the lamp usually