ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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to sliorten tbe draw-tube before tbo second measure is made. The 
increase in tube-length need not be so great in the case of a short tube 
as in a long. In the actual measures I made of the distance between 
Mr. Smith’s upper fine grating and the ordinary well-known grating on 
P. formosum I found the distance between them, at the original adjust- 
ment, was 1 /8000 in. ; an additional inch of tube on the long body 
increased that distance to 1/5000 in., thus making a difference of nearly 
1/13,000 in. A reference to the subjoined figure will make the case 
abundantly clear. Let AB represent the front lens 
of an over- corrected objective, C being the objective. 
Now when a P. formosum is placed at C, the spectra 
of the focus for the central, and D the focus for the 
marginal portion of the first order only are recom- 
bined at the objective conjugate, consequently only 
the original coarse structure is seen. Let, however, 
the focus be raised so that the valve is placed at D, 
then the spectra of the second order only are united at 
the conjugate, and a grating of double fineness is seen. The move- 
ment of the objective gives, of course, an irresistible idea that the fine 
grating is above the coarse. The effect of lengthening the tube is to 
increase the over-correction of the objective, i. e. the distance between 
C and D, and consequently the distance between the fine and coarse 
gratings. 
Let me put it in another way : when there is, say, an over-corrected 
objective on the Microscope, it is just the same as if there were two 
separate objectives in a rotating nose-piece : the one, a narrow-angled 
lens of short focus, which is only capable of resolving the coarse grating 
of, say, 25,000 per inch ; the other, a wide-angled longer-focused ob- 
jective with its centre stopped out, which exhibits the same grating as 
possessing double fineness, viz. 50,000 per inch. In such a case no one 
would have been led astray, as an alteration of focus would have been 
expected, but because all this occurs in one and the same objective, a 
confusion has arisen. 
It is for those who deny the effect of diffraction in the production 
of the Microscope image, and those who insist on the reality of struc- 
tures which are not consonant with that theory, to explain why an 
alteration of one inch in tube-length nearly doubles the distance between 
the gratings. 
Method for measuring the Spherical and Chromatic Aberration 
of Microscope Objectives.* — M. C. J. A. Leroy remarks that the prin- 
ciple of his method is similar to that which enabled Foucault to put in 
practice his method of “ retouches locales.” The surface of the objective, 
observed through a small hole in a screen is seen illuminated only in 
the part traversed by the rays isolated by the hole. When the object is 
a monochromatic luminous point, and the small hole is on one side of 
the axis, the part illuminated is on the same side or the opposite, 
according as the corresponding rays cut the axis in front of or behind 
the plane of the small hole. If the latter be displaced transversely the 
limit of the bright zone will be displaced, under the same conditions, in 
Fig. 28. 
Coniptes Rend us, cix. (1889) pp. 857-9. 
