170 Tranmdions of the 



test, though I feel confident to reach it also. However, we have 

 heard that in America Drs. Woodward and Curtis did already suc- 

 ceed in resolving those mimitissimee strife ; and to prove that what 

 they have seen was no delusion or hallucination, they have got 

 exact photographs of them, copies of which they sent over to 

 Europe. 



After this, I think we may safely assert that, even as it is now, 

 the microscope has fairly reached — nay, we might say has passed — 

 the line which Professor Huxley deemed indispensable for his histo- 

 logical purpose. 



Another proof I might adduce in confirmation of what I have 

 stated — that is a photograph, which I have obtained last year, of 

 Amphipleura ijeUucida, out of Moller's graduated test-plate, not- 

 withstanding the difficulties which attended that preparation on 

 balsam. Unfortunately my negative was blurred and rather faintish, 

 so that it could not give good positive images. Nevertheless, the 

 striae are there so finely and so distinctly drawn out, that they may 

 be perceived clearly enough, though the magnifying power of the 

 microscope was not higher than 640 diameters. The matrix gives 

 without any doubt 4000 in a millimetre, which answers completely 

 Huxley's desideratum of one hundred thousandth of an inch. It 

 was Hartnack's No. 10 that gave me at first that photograph, the 

 illumination being taken fi'om a decomposed sunbeam. But I had 

 afterwards the same result with the same number of Nachet ; both, 

 doubtless, first-rate objectives, since they stood the most diflicult 

 trials. However, as I am desirous to advance scientific inquuies, 

 rather than to boast of my own instruments, I dare say that, should 

 Professor Huxley or any other student of the secrets of nature, try 

 the same thing with microscopes of the best make, either European 

 or American, using the same precautions which I did, they would 

 hardly fail to obtain the very same results. 



My practice is this : whenever I want to make out some of the 

 minutest details of any organism, or to get over any difficult test, 

 and I see that my microscope, after all due preparation, and with 

 the best prospect of hght, fails to answer my expectation, I refer, 

 as a last resource, to my prism, and get from it a coloured sunbeam. 

 Blue or green are the colours which I prefer; they are the most 

 suitable for the purpose. 



The ehmination of every, even the shghtest, chromatic aberra- 

 tion obtained by this means increases, in my opinion, the defining and 

 penetrating power of the microscope, and enlarges its dominion on 

 the field of observation. Difi'erent other means have been now and 

 then suggested, such as an alcohol light saturated with chlorine of 

 iodine, or a light passed through a stratum of cupro-ammoniacal solu- 

 tion, or even thi'ough a glass of cobalt : all these lights may be very 

 useful and for some special purpose even preferable to any other, as 



