222 THEORY OF MICROSCOPIC OBSERVATION. 



form, is not possible by mere examination of the objects in water. 

 In most cases, however, means are available which answer our 

 purpose. If we place the object in a medium that refracts the 

 light more strongly than the densest parts of the object for 

 instance, sulphide of carbon the marking will remain essentially 

 unaltered if it is caused by differences of density ; on the other 

 hand, if the cause lies in the unevenness of the surface the 

 marking changes its character, so that the light and shadow are 

 reversed, as in a photographic negative. The pores of a membrane 

 then appear like papillary elevations, fibrous thickenings, or 

 crevices, and are reversed ; we might regard the whole image also 

 as a negative. 



The explanation of these phenomena is evident. If the object 

 is bounded by plane surfaces, so is also the surrounding sulphide 

 of carbon. The latter therefore acts just as a plate of glass of 

 corresponding thickness it raises the object points without altering 

 their images. If there are, on the other hand, depressions or 

 elevations on the upper or lower surface of the object, at the surface 

 of contact of the surrounding medium the opposite relief occurs, 

 and since this latter is denser it will determine the distribution of 

 light in the microscopic image. The object itself acts optically as 

 a cavity of similar shape in a refracting substance. 



In experimenting with this process, we must not of course allow 

 the object to dry. The loose and dense layers must be saturated 

 with water, otherwise the latter will project outwards, in con- 

 sequence of the greater loss in the aqueous portions, and hence 

 interfere with the optical action. In a perfectly dry state no 

 divisions into separate layers will appear. 



6. VISION THROUGH STEREOSCOPIC BINOCULAR MICROSCOPES. 



The remarkable illusion which stereoscopic contrivances of all 

 kinds produce, is in general a purely psychological phenomenon 

 which we need not discuss. We confine ourselves to the physical 

 question, how the two images which the binocular Microscope 

 presents to the eyes are to be distinguished from one another; 

 whether their mutual relations with reference to the distribution 

 of light and shadow are similar to those which obtain in 

 stereoscopic photographs. If this is really the case, the perception 



