374 POLARISATION. 



object, or is only slightly distant from it. We do not therefore 

 see the real image of the object under examination through the 

 eye-piece, but merely the system of the interference curves 

 which the incident rays produce in the plane of intersection. 



Hence the applicability of the instrument is confined to 

 crystalloid objects or parts of objects, which are large enough 

 for us to be able to observe them separately. As such, for 

 instance, we may name somewhat larger sections of horn and 

 chitine, sections of muscle, cell-membranes of considerable extent, 

 &c. To keep off all stray light, the preparation should be placed 

 upon a small aperture in an otherwise opaque surfaca, for which 

 purpose a thin leaf of tin-foil, for instance, fastened upon the 

 object-stage is convenient. The interference curves which then 

 appear naturally agree with those which very thin plates of 

 crystal produce in the polarising apparatus. As, however, most 

 text-books of Physics treat this point in sufficient detail, it is 

 unnecessary for us to enter into it more fully. 



The great majority of microscopic objects remain inaccessible 

 for the so-called polarising Microscope. Take, for example, 

 any cellular tissue, say a section of fir, where each cell-wall 

 acts as a correspondingly situated plate of crystal, we should 

 in vain try to cover up all the cell-walls to the very last one, 

 in order to observe their interference curves. It would therefore 

 be futile to think of succeeding with cylindrical and spherical 

 structures, whose crystalloid elements are much smaller than 

 those in prismatic wood-cells ! 



The observation of cellular tissue in the polarising Microscope 

 can lead to a successful result only if certain walls are so 

 preponderantly represented, that in sections made parallel to 

 them they influence the optical effect i.e., give rise to phenomena 

 which are not essentially disturbed by the other walls. 



Pardon Son*, Printers, Wine Office Court, Fleet Street, London, E.G. 



