CORRESPONDENCE. 
41 
Immersion Objectives : their Aperture. 
To the Editor of the ‘ Monthly Microscopical Journal .’ 
Regent Street, June 21, 1872. 
Sir, — A contributor to tbe £ M. M. J.’ of May, 1872, who seems to 
me to have an unworthy habit of introducing personalities, has picked 
up at second band, without acknowledgment, a criticism of Dr. Royston- 
Pigott’s,* which I should not care to notice, but that I am reminded 
by Mr. Jabez Hogg that I omitted to say a few words of explanation 
in answer to Dr. Royston-Pigott. 
In October, 1868, I addressed a communication to the Royal 
Microscopical Society with special reference to the subject of immer- 
sion objectives. I ventured to suggest that “ the limiting angle of 
refraction in water being about 48°, it follows that whatever is the 
degree of obliquity in the incident light on the object, the immer- 
sion objective never has to do with rays of greater obliquity than 
48°.” The “ limiting angle ” referred to is evidently the limiting 
angle of transmission between water and air, therefore I was referring 
to the condition of the rays from the dry object at their incidence on 
the immersion objective. That I did not say this specifically was 
doubtless the cause of Dr. Royston-Pigott’s criticism. I may add 
that, save by Dr. Royston-Pigott and the contributor above referred 
to, I am not personally aware that anyone interested in the subject 
found a difficulty in interpreting my observation. 
Dr. Royston-Pigott has not upon all occasions expressed himself 
with that accuracy which is desirable. For example, in ‘ M. M. J.’ 
No. XIX., j" speaking of the rays emanating from a luminous point in 
balsam, he says that “ every ray .... up to 62° 57' will be able to 
reach the facet lens of the objective via water.” But his own diagram, | 
however inaccurate it may be as a representation of an immersion front 
designed to transmit very oblique rays, is accurate as against this 
statement of his, for he exhibits several rays less oblique than 62° 57' 
in the balsam which emerge into the water film and are there lost ; they 
certainly are not drawn as “reaching the facet lens of the objective.” 
Again, in the same number of the ‘ M. M. J.,’ p. 138, Dr. Royston- 
Pigott says : “ Whenever amongst several parallel plates the ray is 
examined , its direction in two identically refractive media is always found 
to he the same [sic] after any number of intermediate refractions .” 
In the standard works on Optics quoted by Dr. Royston-Pigott, 
namely, Griffin, Parkinson, &c., the direction of these rays is not 
described as “ the same,” but as “ parallel.” If I am not mistaken, 
“ the same ” means one thing, and “ parallel ” means another. 
But these are only slight blemishes in these two interesting papers 
on “ Immersion Lenses,” which I should not have noticed but that I 
am advised that I am bound, if possible, to exhibit a weak point in 
my critic’s own positions if only to show that I have read his attack 
on myself. 
* ‘ M. M. J.,’ Nu. xxvi., Feb., 1871, p. 71. 
f July, 1870, p. 23. 
| ‘M. M. J.,’ No, xxi., Sept., 1870, facing page 131. 
