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CHAPTEE I 



ILLUMINANTS 



Oil, incandescent gas, lime-light, and electricity have all been pressed into service. 

 The first is a poor light, and when used necessitates a long exposure even with mag- 

 nification of 200 to 300 diameters, but may be advantageously employed when using 

 quite low-power work. Double wicks are to be avoided ; one large wick used edge- 

 ways, excepting under special circumstances, is found more suitable. Camphor mixed 

 with the oil— the amount seems of but little moment— is used to remove a large 

 amount of the smell, but the best method really of getting over this trouble is to see 

 that the lamp is scrupulously clean. For that purpose, the wick being removed, the 

 whole lamp should be placed in absolutely boiling water and left for a few minutes. 

 After submitting it to a repeated washing, it should be well dried and the wick kept 

 away in a stoppered bottle. When the lamp is now used, after these repeated 

 washings, if no oil be spilled over any part of it in filling, there will be but little smell, 

 and what is then left the camphor mostly removes. 



Incandescent gas is very good for low-power work, but is objectionable on account 

 of the fact that if "critical light" be used, as explained hereafter, on focussing the 

 mantle its uneven appearance is added to the picture, which, save under very excep- 

 tional circumstances, spoils its beauty. 



But oil as well as incandescent gas are not to be recommended in high-power work 

 for another reason. Both theory and practice demand the use of an illuminant for 

 critical work with as near an approach as possible to a point of light. For the 

 optical student this is easy to understand, but if the reader has any misgivings on the 

 philosophy of the assertion, he can easily satisfy himself as to its correctness by con- 

 sulting any elementary work on optics, and the formation of the optical image in the 

 microscope. 



Electric light on this score, however, offers the exact ideal of the photo-micro- 

 scopist, as, of course, with the arc form, a point of light may be said to be continually 



