ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 769 



here. The eye can only see under certain very definite conditions. 

 There must be a definite amount of light in the retinal image or the 

 optic nerves will not be at all affected. Hence a very swiftly-moving 

 object which sends from any one position light for an infinitesimal 

 instant only, is invisible, or is seen only as a blur. So one that is 

 quite at rest may send too little light and be unseen. But the eye 

 whose retina is gelatine and silver bromide, can be made so quickly 

 sensitive that it can catch with ease the swiftest leap of greyhound or 

 racehorse, or the still swifter, though far remote, uprushings of the 

 great fire-clouds of the photosphere ; or it can be made so sensitively 

 slow, that it will gather in for hours the dim light that comes from 

 the distant star-depths, and build up by slow degrees an image that 

 the eye alone could never see. So, may not photography compass the 

 same results in microscopic work ? In high amplification the loss of 

 light becomes soon a limiting value to the possibility of ocular vision, 

 and all details are lost in dimness, but the gelatine plate can be made 

 to take its time to it, as the eye cannot, and slowly gathers up out of 

 the thick darkness an image for our study, if only we can correct and 

 focus properly. It may not be even swift enough to follow the mole- 

 cule or atom ia its flight ; but there are other motions, now in dispute, 

 that it may yet be made to seize, the waving cilia, the yet unseen 

 motile organs of the diatom, the flagella of the bacterium, and still 

 others yet unknown. Still more, it is not impossible that photography 

 may verify exceedingly minute structure in another way — by sub- 

 jecting the details of the photographic image to further enlargement. 

 To make the process of service in this direction, however, will demand 

 a much greater perfection of manipulation than in other departments 

 of photographic work, where it has been successfully employed, and 

 whether it can ever give a true image of details finer than the limit 

 of visibility is, I think, doubtful, in spite of Prof. Abbe's seeming 

 indorsement of its possibility in the article in ' The Monthly Micro- 

 scopical Journal ' of November, 1875. Il is, however, well worth the 

 thorough trial. 



4. Media and Eeagents. — A wise and careful use of the diverse 

 chemical fluids which have, of late years, been brought into notice, 

 will form the most efficient means of verification. I have already 

 referred to the large part that the preparation of an object has to do 

 with its successful microscopic examination. The dilferent media 

 that have been proposed from time to time for preparing objects, for 

 permanently mounting them, and for various test reactions upon them, 

 are almost endless. But of late years there has been a more in- 

 telligent application of chemistry and chemical physics to the aid of 

 microscopic investigation, the principles involved are better under- 

 stood, and we are now armed as never before, with means of putting 

 nature to the test and verifying our vision of her most intricate 

 minutiae. Yet many microscopists work on in old ruts, mounting 

 everything in one and the same medium. Some look on staining as 

 only a refinement of dilettantism, a thing of mere looks, like coloured 

 varnish rings and ornamental labels. But these staining fluids, as 



