196 



THE AMERICAN MONTHLY 



[October, 



dent of science should be quick to 

 take advantage of every aid afforded 

 by the optician's skill ; as to the 

 practical value of the Abbe condenser 

 there can be but one opinion. 



Reasoning from what has been said 

 concerning the theory of the micro- 

 scope, it w^ill be obvious that a careful 

 management of the illumination is 

 essential to the best results in micro- 

 scopical w^ork. The mirror alone 

 can be made to do almost everything, 

 but it must be handled with far more 

 skill than most scientific observers 

 possess. They do not acquire such 

 skill in the work that occupies their 

 attention, and for that reason they 

 fail to use the microscope to the best 

 advantage. Not only is this true, but 

 their experience as observers is often 

 too much restricted, and I have known 

 persons of undoubted ability in cer- 

 tain lines of work to make surprising 

 blunders, due to misinterpretations of 

 images in the microscope, which no 

 person of broader experience would 

 make. It is a great advantage to any 

 observer to be able to resolve fine 

 test-objects, as such work gives not 

 only command over the instrument, 

 but it teaches one how to get the 

 clearest definition, and how to avoid 

 or detect false appearances. The 

 use of the Abbe illuminator greatly 

 facilitates the management of the 

 light, and often brings out details of 

 structure very clearly that would 

 otherwise be very easily passed with- 

 out notice. 



The use of polarized light in con- 

 nection with the microscope is of 

 great importance to the mineralogist, 

 who is enabled to study the optical 

 properties of minerals as they occur in 

 rocks in mici'oscopic quantities. The 

 ordinary arrangements for polarized 

 light serve well enough for such pur- 

 poses and for the ordinary require- 

 ments of the microscopist. A far 

 more elaborate and delicate apparatus 

 for studying the optical properties of 

 minute objects is the polari-spectro- 

 microscope of RoUet*, or the spectro- 



* Am. Monthly Micr. Journ., iv, i68. 



polarizer of Zeiss * which serves the 

 same purpose and can be attached to 

 the ordinary microscope. Both these 

 instruments consist of a polarizing 

 prism, a dispersing prism to form a 

 spectrum of the polarized light, and 

 a plate of selenite. The different 

 colors of the spectrum can be made 

 to traverse the field of view in suc- 

 cession. The selenite produces dark 

 bands of interference, the position of 

 which depend upon the thickness of 

 the selenite. An object possessing 

 the slightest property of double re- 

 fraction, such as a piece of muscle, 

 for example, when placed in one of 

 the dark bands adds to the thickness 

 of the selenite, and, in certain azi- 

 muths, becomes luminous in the dark 

 band. These instruments are, un- 

 doubtedly, the most delicate means 

 we have of studying the optical prop- 

 erties of minute objects, and their 

 value is enhanced by the ease with 

 which the wave-lengths of the light 

 used can be measured. 



The relation between resolution of 

 fine lines and the wave-length of light 

 makes it obvious that with blue light 

 finer details can be seen than with 

 light of lower refrangibility ; also, as 

 already said, that finer details maybe 

 photographed than can be seen. The 

 usual method of obtaining monochro- 

 matic light is by the use of a colored 

 glass or liquid interposed in the path 

 of the rays. A more elaborate device 

 has been constructed by Mr. Zeiss. 

 In this, two prisms are mounted as 

 for a spectroscope in such a manner 

 as to throw a spectrum upon the ob- 

 ject under the microscope, which may 

 then be examined in light of any color. 



Method of Demonstrating the 

 presence of the Tubercle Bacil- 

 lus in Sputum. 



BY THEOBALD SMITH, M. D., A. M. 



In the examination of sputum for 

 the Bacillus tuberculosis^ it is es- 

 sential that the sputum be from the 

 proper source, that the method of 



* Loc. Cit., iv, 174. 



