32 Transaetions of the Society. 



As the source of light is almost a point, and the lamp can be 

 brought very nearly into contact with the slide, a greater degree of 

 obliquity of the illuminating rays can thus be obtained than by 

 almost any other method, and hence black-ground illumination is 

 shown with great beauty, and many of the diatoms display diifrac- 

 tion colours with unusual splendour. The resolution of test objects 

 becomes very much simplified, as most of them can be resolved by 

 the lamp alone, without any accessory apparatus. 



(3) For use with the polariscope, a third lamp C, of slightly 

 larger size, is placed in the position of the usual mirror. It is 

 put in action by moving the switch to the left, so as to make 

 contact with the third stud. This lamp requires an additional 

 cell so as to develope a light of about four candles. 



As the sockets of the lamps are all made to a standard size, it 

 is easy if more light be required than is given by the smaller 

 lamp, to transpose the larger one to either of the other positions 

 and use the full strength of the battery. If it is found desirable 

 with the lower powers to give parallelism or convergence to the 

 rays, a very small lens can be mounted in front of the lamp. 



If a more simple mounting is desired, the forms shown in figs. 

 4, 5, and 6 may be adopted ; and the lamp can be thus placed in 

 any position above or below the stage. 



If it is required to maintain the lamps for several hours at 

 full incandescence, the most satisfactory battery to use would 

 undoubtedly be a Bunsen or Grove. If, however, the switch is 

 turned ofi" whenever an observation is completed, a recent modifi- 

 cation of the Leclanche answers admirably ; for if exhausted through 

 polarization it recovers itself when left for a short time, and will, 

 when once filled, keep in good order for several months. It is best 

 to use five of these modified Leclanche cells, controlling the strength 

 of the current by means of the resistance, and diminishing it as 

 the potential of the battery falls. For all ordinary work these 

 Leclanche cells will be found to meet all the requirements of the 

 microscopist. The Swan-Sellon, or Faure accumulator will also 

 be found convenient, but these are at present rather expensive 

 luxuries; and though they last for a considerable time when 

 charged, the trouble of charging at intervals would probably 

 counterbalance the advantages gained in other ways. 



I have been able to light these lamps satisfactorily with a small 

 dynamo, about five inches in length; and if it be possible to obtain 

 a spring which can be wound up by hand, and will drive it for 

 about half an hour without occupying too great a space, this may 

 probably be a very convenient method of obtaining the current when 

 required. "When, however, we consider that to obtain the amount 

 of electrical energy represented by the product of 3^ volts and IJ 

 amperes, we should have to expend about 4 or 5 foot-lbs. of mechanical 



