ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
519 
rated by their maker (Spencer) as having N.A. 1*32 and N.A. 1*38, 
were next tried with monobromide, and found to respond to it perfectly, 
it being necessary, as with the others, to close the systems and shorten 
the tube-length. I carefully measured the angular aperture of one of 
these with water, oil, and with monobromide; I found that its nume- 
rical aperture with water was 1*22; with oil, 1*39; and with mono- 
bromide, 1*51” 
On this quotation it may, Dr. Dallinger remarked, be said that 
by the use of pure monochromes of the spectrum we can with our 
achromatic lenses get what is, in effect, a much larger N.A. than the 
monobromide will represent ; and we do not despair of the ultimate use of 
such a monochrome as will bring the N.A. in practice up to 2 * 00, or even 
2*20, and this will be done with media much more tolerant of organic 
tissues than the monobromide. Moreover, there is great uncertainty in 
the results obtained with this latter medium ; it is very uncertain with 
different lenses presenting no normal reason for such difference ; and 
we have in no case obtained what is in an eminent sense a “ critical 
image.” 
Researches on the Optical Properties of Wood.* — M. C. Houlbert 
describes the diffraction phenomena exhibited by thin sections of wood, 
.ind makes use of them in order to determine the dimensions of the fibres. 
In thin tangential sections of wood the opaque walls of the fibres, 
disposed longitudinally, together with the alternating clear spaces in 
the interior of the fibres, constitute a system of lines which acts as a 
diffraction grating. 
The following phenomena (fig. 50) are exhibited on looking at a 
vertical luminous slit across a thin tangential section of sideroxylon 
Fig. 50. 
(S. laurifolium) placed in such a way that the axis of the fibres is parallel 
to that of the slit : — 
(1) The luminous slit appears very bright, with luminous bands to 
right and left, diminishing in brightness as they separate from the centre. 
(2) On each side of the central slit and separated from it by a black 
band, are two very narrow brilliant lines which represent really two very 
narrow spectra. (3) Next to these lines comes a black band of slight 
width. (4) A clearly visible, but narrow spectrum. (5) A black band. 
(6) A spectrum of appreciable width, in which all the fundamental 
* Rev. Gen. de Bot., xvi. (1891) pp. 49-59 (5 figs.). 
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