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SC MM ARY OF CURRENT RESEARCHES RELATING TO 
the writer, lie would have certainly suggested the obvious method of 
softening down the intensity of the flame-image by interposing coloured 
screens. Taking the whole tenor of the passage, there caunot be the 
bast doubt that the ends sought for were suitable intensity of light and 
equable illumination of field ; the frequent mention of the word cone 
being more accidental than insisted on for the sake of the cone itself. 
It is as a cone-producer wherein the efficacy of the condenser lies. 
If, as is implied in the text-books, it were only light-intensity which 
gave criticalness to the image, that could be secured by exchanging the 
light from the 1/2-in. paraffin wick for that from the electric arc, but 
such an exchange would cause no alteration in the character of the 
image so long as the aperture of the cone remained the same. 
The real office of the substage condenser being a cone-producer, the 
first question that arises is, What ought to be the angle of the cone ? 
This is really the most important question that can be raised with 
regard to microscopical manipulation. To this I reply that a 3/4 cone is 
the perfection of illumination for the Microscope of the present day.* By 
this I mean that the cone from the condenser should be of such a size as 
to fill 3/4 of the back of the objective with light, thus N.A. 1*0 is a 
suitable illuminating cone for an objective of 1 * 4= N.A. (dark grounds 
are not at present under consideration). This opinion is in direct 
opposition to that of Prof. Abbe in his last paper on the subject in the 
December number of the B. M. S. Journal for 1889, where he says : 
— t; The resulting image produced by means of a broad illuminating 
beam is always a mixture of a multitude of partial images, which are 
more or less different (and dissimilar to the object itself). There is 
not the least rational ground — nor any experimental proof — for the 
expectation that this mixture should come nearer to a strictly correct 
projection of the object (be less dissimilar to the latter) than that 
image which is projected by means of a narrow axial illuminating 
pencil.’ - f 
This paper I consider to be the most dangerous paper ever published, 
and unless a warning is sounded it will inevitably lead to erroneous 
manipulation, which is inseparably connected with erroneous interpre- 
tation of structure. 
If you intend to carry out his views and use narrow-angled cones, 
you do not need a condenser at all — more than this, a condenser is 
absolutely injurious, because it affords you the possibility of using a 
large cone, which, according to Prof. Abbe, yields an image dissimilar 
to the object. If there is the slightest foundation for Prof. Abbe’s 
conclusion, then a condenser is to be avoided, and when a mirror is used 
with low powers care must be exercised to cut the cone well down by 
the diaphragm. 
In the opening sentence of the paper Prof. Abbe says, “ The diffraction 
theory leads to the following conclusions in regard to the mode of 
illumination in question.” We are, therefore, thrown back on the 
diffraction theory, for the discussion of which I must ask your kind 
* Mr. Comber (R. M. S., May 21st, 1890) states that in practice he finds a 
2/3 cone best for photomicrography. A 2/3 cone (photographically) is to a 3/4 cone 
(visually) as 10/12 is to 9/12. Mr. Comber’s experience is therefore in accordance 
with this statement. t R.M.S. Journal, 1889, Part 6, p. 723. 
