Dallinger, Drysdale's u. Dalliuger's Untersuchungen an Biflagellaten. 309 



of course consecutively as with the dry lenses - - but in isolated detail 

 and confirmed our proceeding work making- new drawings. 



We found the enlarged numerical aperture, and the correction of 

 the spectra not corrected by the achromatic objective -glasses of 

 inestimable value; but we found that the use of a suitable condenser 

 with N. A from TO to 1'5 absolutely indispensible for the best results. 



It will thus be seen that the details of the work in question, and 

 consequently the details of the life -cycle of this ,,biflagellate" form 

 have been gone over several times since they were first done, and 

 with lenses and entire optical systems wholly different from those 

 employed at the beginning. 



As I have practically followed the improvements in the optical 

 apparatus of the microscope for thirty years, and have during all that 

 time cooperated with some of the best manipulators in the world, I 

 naturally attach importance to apparatus, and method. Moreover 

 I find myself greatly doubting whether the ,,nioist chamber" employed 

 by France would in my hands have been satisfactory for doing the 

 above work. But its use is probably made more efficient by the 

 statement of France on p. 297 viz that ,,all the researches here pre- 

 sented were carried out partly and indeed mostly on living 



material, partly however also on preserved and stained 

 material". This of course leaves us in doubt as to how far essential 

 points were inferred, not only as phenomena, but as sequences from 

 post mortem and stained specimens; but even more, it leaves us in 

 doubt as to how far the absence of certain phenomena, which in 

 my experimental experience, would be most certainly absent from, or 

 indiscoverable in ,,preserved and stained material" of the ,,biflagellate" 

 monad, is to be attributed to the use of such material. 



That there are many biological subjects and even very minute 

 biological subjects that will tolerate the process of drying without 

 much prejudice to the discovery of accurate results I do not for an 

 instant doubt; but this does not apply to the septic organisms. Myself 

 and colleague obtained the strongest evidence as I have already hinted, 

 from experience and experiment, that errors, negative and positive, 

 are inevitable to a study of these minute forms in any way dependent 

 on drying and staining. But I venture to think that the errors resul- 

 ting from its employment are rather multiplied than diminished when 

 the work of observation on the life development of such forms is done 



,,partly on living material, partly on preserved and 



stained material".. 



Observations up to a certain limit on the life -cycles of even the 

 minutest of these and similar organisms are not really difficult, if we 

 possess two or three good microscopes, with lenses of the best correc- 

 tions worked with a suitable condenser, and a well constructed piece 



