CORRESPONDENCE. 105 
forms a separate chapter at the end of his essay, in which Helmholtz 
makes no reference to the formula discussed in the first part. More- 
over, the altered expression of this formula is in no sense a “ popular- 
ization.” For to change the terms of a formula, as in the present 
case, is not to make it less recondite, but to give it a more subtle 
expression, and a more precise definition. Such formule are yet 
remote from popular thought ! 
The cases of diffraction caused by interception of light by close- 
drawn lines (as in ruled test-plates) or minute particles in an object 
(as in all substances exhibiting minute details of structure) have been 
specially discussed by Professors Helmholtz and Abbe, and the con- 
sideration of such cases led the latter named gentleman to an experi- 
mental verification of the formule worked out by the Professors 
separately. This experimental verification I have myself, through 
the kindness of Professor Abbe, witnessed, and I propose to give a 
short account of the method adopted and its result in the course of a 
few weeks. 
Finally, it may be remarked that Fraunhofer’s conjecture “ that 
an object of less linear diameter than a wave-length can never be dis- 
cerned by microscopes as consisting of parts,” which is doubted by 
Sir J. Herschel, as not being a necessary conclusion from the pre- 
mises, is fairly met by the mathematical formula which places the 
limit of visibility at half the wave-length, «= A,, when the divergence 
angle a, is equal to 90° (that is, with employment of immersion 
objectives). See Helmholtz ‘On the Limits of Optical Capacity of 
the Microscope.’ 
I am, Sir, yours faithfully, 
H. E. Fripp. 
VARIATION IN NAvICULA RHOMBOIDES. 
To the Editor of the ‘ Monthly Microscopical Journal.’ 
Sir,—On reading the excellent paper by the Rev. W. H. Dallinger 
ou the forms and striation of N. rhomboides in last month’s Journal, 
one is struck with the evident pains the author has taken to make 
distinct and generous reference to those writers on his subject who 
preceded him. It is therefore most certain that only by an oversight— 
perfectly excusable by reason of the voluminous literature of the 
diatom—he has omitted to notice the work of Mr. William Hendry, of 
Hull, who, some fifteen years ago, carefully examined and measured 
many varieties of N. rhomboides, and came to the conclusion that the 
species “is the most variable in its dimensions, irregular in its form 
. and possesses a striation more extended in its range than any 
other known diatom, thus totally unfitting it to take rank, under any 
circumstances, as a test-object.’ * In one direction he even went a 
step beyond later writers, by showing, in his tabulated measurements, 
that “the numerical striation bears no definite relation to the magni- 
* ‘Quarterly Journal of Microscopical Science’ for 1861, p. 231. 
