THE BINOCULAR MICROSCOPE. 121 



ance; but in proportion as the grains are smaller, so are their mark- 

 ings and colourings less distinct. 



" The application of this modification of light to the illumination 

 of very minute structures has not yet been fully carried out j but still 

 there is no test of differences in density between any two or more parts 

 of the same substance that can at all approach it in delicacy. All 

 structures, therefore, belonging either to the animal, vegetable, or mi- 

 neral kingdom, in which the power of unequal or double refraction is 

 suspected to be present, are those that should especially be investigated 

 by polarised light. Some of the most delicate of the elementary tis- 

 sues of animals, such as the tubes of nerves, the ultimate fibrillse of 

 muscles, &c. are amongst the most striking subjects that may be studied 

 with advantage under this method of illumination. Every structure 

 that the microscopist is investigating should be examined by this light, 

 as well as by that either transmitted or reflected. Objects mounted in 

 Canada balsam, that are far too delicate to exhibit any structure under 

 transmitted, will often be well seen under polarised light ; its uses, 

 therefore, are manifold."* 



APPLICATION OF BINOCULARITY TO THE MICROSCOPE. 



The application of this principle to microscopic purposes seems to 

 have been tried as early as 1677, by a French philosopher, le Pere 

 Cherubin, of Orleans, a Capuchin friar. The following is an extract 

 from the description given by him of his instrument : " Some years 

 ago I resolved to effect what I had long before premeditated, to make 

 a microscope to see the smallest objects with the two eyes conjointly ; 

 and this project has succeeded even beyond my expectation, with ad- 

 vantages above the single instrument so extraordinary and so surpris- 

 ing, that every intelligent person to whom I have shown the effect has 

 assured me that inquiring philosophers will be highly pleased with the 

 communication." 



This appears to have long slumbered and been forgotten; and 

 nothing more was heard of the subject until Professor Wheatstone's 

 very surprising invention of the stereoscope, when it again attracted 

 the attention of the above philosopher, who applied to both Ross and 

 Powell to construct him an instrument. But this was not done ; and 

 during the year 1853 a notice appeared in jSillimans American Journal 

 * Quekett's Practical Treatise on the Use of the Microscope. 



