30 Transactions of the Societij. 



large size,* the battery power necessary to render them incandescent 

 would, till electricity is supplied from a central station, constitute 

 a bar to their general use. 



There can be no advantage in using a large light at a distance 

 from the object, when a small one near to it will give as good, or 

 better, results, and will at the same time require the expendi- 

 ture of so little electrical energy, that the trouble attendant on 

 the use of the battery is almost inappreciable ; and in this way 

 the lamp can be made a peimanent attachment to the Microscope 

 itself. 



The lamps I have constructed for the purpose are shown full 



F 9 ^^^® ^ ^S^* ^ ^^^ ^' ^^^ ^'^ ^^'^^ ^"^ ^^^ Micro- 

 scope in fig. 3 at A B and C. The length of 

 the incandescent filament is 1-1 0th of an inch, its 

 diameter l-16b'th of an inch, and its superficial 

 area about 1-5 5 5th of a square inch. Two 

 Bunsen or four Leclanche cells are sufllcient to 

 render them fully incandescent ; but for general 

 purposes it will be best to use an additional ceU, 

 regulating the intensity of the light by means 

 of the adjustable resistance coil D interposed in the battery circuit 

 and attached to the base of the Microscope. 



As the duration of the lamps is in an inverse ratio to the 

 temperature at which they are maintained, it is desirable that the 

 most intense light that the lamp will give should only be employed 

 for a very short time when a special efi'ect is required ; such, for 

 instance, as for purposes of micro-photography. If the lamp is at 

 other times used no brighter than is necessary to obtain a white 

 light, and the current turned ofi" when observation is not going on, 

 the lamps will last a very long time, as experience has shown that 

 a hfe of more than 2000 hours of continuous and brilliant incan- 

 descence is frequently exceeded by Swan lamps. It is possible to 

 obtain a hght of 2^ candles fi'om the tiny surface just mentioned, 

 with an electro-motive force of 3i volts, and a current of IJ 

 amperes. It would, however, at a safe temperature, give a light 

 equal to one candle. 



It will be found the most convenient plan to keep more than 

 one, say three, of these lamps on the instrument, so that by merely 

 turning a switch the position of the light may be varied. 



(1) For the illumination of opaque objects the lamp A (fig. 3) 

 is attached by a jointed arm E, to an insulated collar a, which 

 screws on above the objective. The source of hght can then be 

 rotated around the object while under examination, so that delicate 

 surface markings can be readily brought out. 



* Since the above was written Dr. Van Heurck has informed me that he has 

 used lamps requiring an electro-motive force of not more than 7 volts. 



