ON THE THEORY OF THE MICROSCOPE. 243 



deduce I by calcuiation without any necessity for knowing the 

 aotual structure of the object. 



That the same state of things obtains in numerous instances of or- 

 ganic forms, the study of which belongs to the province of Histology, 

 we may learn from the instance of striated muscular fibre. In 

 good preparations the diffraction phenomena are readily observed 

 and may be studied experimentally by the methods already 

 described. The manifold changes in the characters of the images 

 which then present themselves account, to a certain extent, for 

 the notorious discordance Detween the representations ol different 

 observers, and also attest the impassibility of interpreting in any 

 satisfactory manner the material composition of this tissue, in the 

 sense in which it has been hitherto attempted. What lias been 

 here urged respecting the principles upon which microscopic vision 

 depends applies further, not only to the morphological relations of 

 objects, but, in quite as great a degree, to other propej'ties, con- 

 cerning which microscopic observation is expected to afford 

 correct conclusions. That many distinctions of transparency and 

 of colouring perceived in the microscopic image do not necessarily 

 indicate any special character of the object, but often arise from 

 partial or entire exclusion of diffraction pencils, is a fact 

 sufficiently illustrated by the known appearances of diatom 

 valves. It seems also of importance to note that the signs of 

 polarisation in the images of objects containing minute detail 

 must be in many respects differently interpreted from the 

 polarisation effect in purely geometrical or " absorption " images. 

 To make inferences, in an ordinary sense, concerning the double 

 refraction of substances is, to say the least, full of hazard ; for 

 there remains always the possibility that the same textural 

 peculiarities which call forth diffraction, may, under circumstances, 

 originate at the same moment polarisation effects, which, so far 

 as these are attributable to the function of diffraction, do not 

 depend, as in crystals, starch, granules, &c., &c., upon peculiar 

 transmission of light rays. That something of the kind does 

 actually occur appears probable from what I have seen when 



