308 THE NORTHERN MICROSCOPIST, 

objective, to 146,528 the maximum of an oil-immersion objective, 
the illuminating power being also increased from r to 2.25: while 
as regards the second point, we are able by the homogeneous-im- 
mersion method to reduce the problem of correcting a very wide- 
angled objective to the much less difficult one of correcting an 
objective of moderate air angle. Our lamented President, the 
Rev. J. B, Reade, declared in 1870 that “the ghost of aberration 
will never be entirely exorcised even by cold water.” But there 
appears to be good ground for believing that oil has practically 
accomplished that object. 
During the past year several kinds of fluids for homogeneous 
immersion have been brought before the Society, such as chloral 
hydrate and glycerine, iodide of zinc and glycerine, and gum 
dammar and cedar-oil. Two other vegetable products have also 
reached us, “‘tacamaque” and the gum-resin “oliban,” or “incense,” 
both dissolved in cedar-oil. While the dammar is claimed to be 
unchangeable, and to be in refractive and dispersive powers very 
near that ideal of a good immersion medium, “ fluid crown glass,” 
there is evidently room for further research in this direction, 
particularly for a fluid which will not attack the various varnishes 
in ordinary use. 
Lastly must be noted an important advance in practical manu- 
facture by the construction, by Messrs. Powell and Lealand, of a 
homogeneous-immersion objective of the large aperture of 1.47 
N.A. out of a possible 1.52. As long ago as 1850 one of my pre- 
decessors in this chair, expressed the belief that objectives had 
then “nearly, if not quite, attained the limit of perfection,” and 
whilst it will be prudent even at this much later date to avoid any 
assertion of finality in the present, or scepticism as to the possi- 
bilities of the future, it must be admitted that so far as regards 
aperture and resolving power we have arrived at a point beyond 
which it will, to all appearances, be difficult to advance, at any rate 
not without serious restrictions in the use of the objectives. Whilst 
it might be possible to work front lenses for objectives out of 
diamond and so to increase the aperture to 2. 5 N.A. and the 
resolving power to 241,000 lines to the inch, it must be remembered 
that it would be essential at the same time to provide an immersion 
fluid, slides, cover-glasses, and illuminators of the same refractive 
index as diamond also, 
Penetrating Power of Objectives—Depth of Vision. 
This again is a subject which has long been obscure; very 
various opinions being held as to the true nature of what has been 
generally termed the “penetrating power” of an objective. By 
some it has been declared to be a defect in the construction of the 

