238 SELECTION AND USE OF THE MICEOSCOPE. 



the metal; two said they were raised. In objects too small to 

 be felt, and where sections cannot be made, the truth may be 

 ascertained by watching the effect of raising or lowering the 

 object glass in focussing. 



Another fallacy of this kind has led to the belief that hairs 

 and many similar bodies are hollow. Seen under the micro- 

 scope, a hair looks just as if it were a tube but then, so does 

 a wire, which is known to be solid. The test in this case is to 

 make a cross section of the object. 



The true form of objects may frequently be determined by 

 studying the effect of light and shade produced by sending 

 the light across them in different directions. This is most 

 readily effected by means of the revolving stage, which, for this 

 purpose, should have very accurate rotation in the optic axis. 



The most singular fallacies, however, are those arising from 

 certain illusions of vision, which affect every one, and which in 

 ordinary practice, are easily corrected. For a full account of 

 these, the reader is referred to the works of Helmholtz; a brief 

 account of the most common cases is given in the Young Scientist 

 for 1881. The one which is of most interest to microscopists is 

 the famous optical illusion of Nachet, of which a figure is given 

 below. In the course of his examination of the markings on the 



Fig. 81. NATCHET'S OPTICAL ILLUSION. 



P. Angulatum, M. Nachet found that if a series of round black 

 dots be arranged on a white ground, as in Fig. 81, the dots, 

 when viewed from a distance of twelve to twenty inches, will 

 appear to be hexagonal, though we know that they are round, 



FINIS, 



