1897-98-] Microscopy and some of its Uses. 317 



is said about illumination. Like the kindred matter of the 

 performance of objectives, illumination is generally measured 

 just as heat was measured before there were any thermometers, 

 by sensation. It is mixed up with the personal equation. 

 Statements about it need to be critically examined. When 

 we give an opinion on illumination, or the performance of an 

 objective, it should always be accompanied by a comparative 

 estimate of the sensitiveness of our eye, and the several other 

 factors in the process. A statement without this is of little 

 value, and may be only misleading. I believe I shall justly 

 epitomise opinion and experience on the subject if I say 

 first, that for all ordinary work the simple mirror is qtiite 

 sufficient ; second, that in some cases the Abbe is useful ; and 

 third, that for special work achromatic illumination is certainly 

 the best. 



After we have made a certain progress in the use of the 

 microscope, while the things we do see fill us with increasing 

 wonder and satisfaction, there is also sometimes a little dis- 

 appointment because of what we have failed to see. We do 

 not at the outset see all the minute details mentioned and 

 figured in the books, and perhaps we blame the lenses, or 

 think we have not the necessary acuteness of observation. 

 But we have to remember that, as in other sciences, these 

 details were only reached by the accumulated efforts of many 

 observers, and that some of them are only to be seen on rare 

 and favourable occasions, and by the use of reagents and other 

 helps. 



Then the results in high-power work are different from 

 what we expected. Our notion of definition, perhaps, was 

 that things should be enclosed by perfectly straight lines and 

 perfect circles and smooth curves, and here we find that there 

 are no perfectly straight lines or perfect circles or smooth 

 curves in nature : that these are only mathematical concep- 

 tions, that the outlines are irregular, and that there is in the 

 works of nature an imperfection analogous to that in human 

 art between the ideal and its embodiment. Some of the most 

 beautiful diatoms, for instance, look best at a power of about 

 250; at 500, signs of irregularity in the markings begin 

 to appear. But this is, in fact, the service wanted from 

 high powers, namely, to make the ultimate structure of 



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