118 SELECTION AND USE 



brass tube, the upper end of which has an external Society 

 screw that attaches it to the body, while the lower end of the 

 tube has an internal Society screw for receiving the objective. 

 Fig. 27 shows the latter arrangement. 



Polarized light, except for the mere beauty of its effects, 

 has not received the attention that it deserves. In some de- 

 partments of scientific investigation, especially mineralogy and 

 geology, its use has afforded very satisfactory and brilliant re- 

 sults. As regards its applications to medicine and physiology, 

 Dr. Frey says: " The examination of tissues by polarized 

 light has a high scientific value, as, by this means, molecular 

 relations become evident, which by investigation with ordinary 

 light, remain entirely concealed. The _ interpretation of what 

 is seen, is in many cases difficult, and generally lies within the 

 province of optics, with which the medical observer is usually 

 but little familiar." 



To detail the method of using it, and the special features 

 which it discloses, would, however, far transcend the limits of 

 this work, and we must, therefore, refer the reader to some 

 special treatise on the subject. 



HOW TO USE THE MICEOSCOPE. 



The remarks which we are now about to offer, are intended 

 for the merest beginners for those, in fact, who have never 

 used a microscope at all; and therefore they may, perhaps, to 

 some, appear childishly simple. And yet we have seen not 

 only teachers, but professors in colleges, v/ho might have de- 

 rived some benefit even from these simple hints. We remem- 

 ber on one occasion seeing a professor of botany attempt to 

 examine a minute plant with a common pocket magnifier with 

 three lenses. In the first place he turned the instrument wrong 

 side up, so that, although he could see through it, the results 

 attained were very inferior to what they would have been if the 

 instrument had been properly used; in the second place he 

 wore his hat in such a way as to cut off nearly all the light, and 

 in the third place he did not know how to hold his hands so as 

 to obtain the requisite degree of steadiness. If he had given 

 a few minutes thought to the subject, he could no doubt have 



