304 Transactions of the Society. 



3. The preceding shows that wide apertures can only be 

 utihzed in the observation of minute details, under high amplifica- 

 tions obtained with objectives of short focal length. Wide aper- 

 tures are therefore useless when those conditions are not fulfilled, 

 because in this case the same result could be obtained as well 

 with low-angled systems. But as abundance prima facie is 

 no detriment, the foregoing considerations do not enforce any 

 positive objection to the use of wide apertures for every kind 

 of work. There are however other points of view from which 

 it becomes obvious that the application of wider apertures than 

 can be utilized is not merely superfluous but is a decided disad- 

 vantage, inasmuch as they prevent the utilization of some really 

 valuable benefits which are the privilege of low and moderate 

 apertures. 



The first disadvantage results from the reduction of the depth 

 of vision (or the " penetration " of the Microscope) which is 

 connected with wide apertures. I have given in another place* 

 a discussion of the circumstances on which penetration depends, 

 and the formulse which afford an approximate numerical estimation 

 of the depth of vision in microscopic observation. These theoretical 

 suggestions show (in accordance with the experience of practical 

 microscopists) the reduction of penetration with increasing aperture 

 under one and the same amplification, and especially when the 

 amplification is not restricted to very small figures. Now there 

 are many objects of microscopical research which do not require, 

 and, indeed, do not even admit of high powers, but demand for 

 effective investigation as much penetration as possible. This is 

 always the case where the recognition of solid forms is of import- 

 ance, and therefore a distinct (at least, a tolerably distinct) vision 

 of diff'erent planes at once must be possible, whether the observa- 

 tion is assisted by stereoscopic devices or not. The greater part 

 of all morphological work is of such a kind, and in this line of 

 observation therefore a proper economy of aperture is of equal 

 importance with economy of power. 



Whenever the depth of the object under observation is not 

 very restricted, and it is essential that the depth dimension shall 

 be within the reach of direct observation, low and moderate powers 

 cannot be overstepped, and no greater aperture should therefore be 

 used than is required for the effectiveness of these powers — an 

 excess in such a case is a real damage. High powers and corre- 

 spondingly wide apertures are restricted to those observations 

 which do not require any perceptible depth of vision, i, e. to two 

 difierent cases (1) when the objects are quite flat or exceedingly 

 thin ; (2) when preparations of greater depth are sufficiently trans- 

 parent to admit of an indirect recognition of their solid structure 



* See this Journal, i. (1881) p. 689. 



