228 Optical Curiosities of Literature. 



In August, 1871, a communication appears from M. Mouchet 

 on the thickness of glass covers. He there describes his method of 

 measuring it, viz. by focussing for the upper and then for the lower 

 surface, and observing how far the tube has been lowered, ascer- 

 tainable by the index of the fine-adjustment screw. This method 

 of taking soundings, as it might be called, is, I believe, discovered 

 independently and used on all occasions by almost everyone who 

 works intelligently with the microscope. It suggests itself, indeed, 

 or rather forces itself on our notice every time we begin to work 

 the screw. It is not, however, for this reason that I notice it, but 

 to point out that M. Mouchet has by an oversight recommended an 

 erroneous measurement with it. The distance traversed by the 

 microscope is not the thickness of the glass. We must in every 

 case correct this distance by increasing it in the proportion of three 

 to two, due to the index of the glass. Of course so long as we 

 only want a correction for the screw-collar the error is of no con- 

 sequence; the proportion of this adjustment to the index of the 

 slow-motion screw remains the same. But if we go outside this, 

 applying it e. g. to the measurement given on some slides by the 

 preparers, in absolute fractions of an inch, then the error will come 

 out : as also when we use it, hke M. Mouchet, to measure the abso- 

 lute thickness of diatoms in balsam. 



In the report of the proceedings of the Eoyal Microscopical 

 Society, in the number for December, 1871, I notice observations 

 of Mr. Slack and j\lr. Brooke on the new lens introduced by Mr, 

 Wenham, which do not correctly state its principle of action. There 

 are two modes of action of which this lens is capable, and these in 

 their nature and conditions are entirely distinct. In the observations 

 referred to (p. 294) these two are mixed up together, so that the 

 meaning of the illumination is lost. This may possibly be due to the 

 necessary compression of the report, the observations having been 

 made in conversation. Taking them, however, as they appear printed, 

 they are not correct, and as they remain still uncorrected I notice 

 them ; principally, however, for the purpose of calling attention to 

 the action, as yet it would seem so little known, of this most peculiar 

 illumination. The persevering efforts which for some time have 

 been made, as yet with no sign of success, to make out disputed 

 structures with very high powers, are forcing us to pause and review 

 our resources. We have in fact come nearly to a standstill, and the 

 indications are, I think, becoming very plain that we can scarcely hope 

 to " force the game," if I may so express it, by increase of power, or 

 even by fineness of corrections alone. For in cases where, the defi- 

 nition being good, the magnification appears amply sufficient, the 

 structure still refuses to reveal itself, and we come to a point where 

 increased amplification yields no increase of knowledge. I have been 

 for some time becoming more and more convinced that we must turn 



