150 
CORRESPONDENCE. 
the two ordinary ones at the base ; but theso extra ones were at the 
extreme tip of the long arms or appendages. Seen thus with four 
balls, they were extremely curious. 
Will Mr. Garner kindly inform us if he ever found the Buce- 
phalus in a free state, and with the characteristic quick movements 
which those I found possessed ? 
Your obedient servant, 
John Badcock. 
On Mr. Slack’s Eeply to Mr. J. Mayall, jun. 
To the Editor of the ‘ Monthly Microscopical Journal.' 
224, Regent Street, London, August 2, 1S75. 
Sir, — As you have allowed Mr. Slack to make criticisms on my 
letter (before its publication), perhaps you will permit me to make a 
few observations in reply to him ? 
Mr. Slack is dissatisfied with my interpretation of his paper on 
“ Angle of Aperture, Ac.” : I was neither so simple nor so sanguine as 
to expect otherwise. I willingly admit that my notions of clearness 
of thought and accuracy of language are often opposed to his. But I 
had no wish to misrepresent him. As 1 read his paper, its general 
tendency — whether avowed or not — was to make the unwary believe 
that an optician had come into the field with new lenses of wondrous 
excellence. He now says the question raised by him “ related to the 
smallest apertures capable of showing lined objects.” Well, the new 
lenses are supposed to be the practical embodiment of the smallest 
apertures capable of showing lined objects ; to criticise them by their 
results was therefore not irrelevant to the question at issue — but the 
contrary. 
He now points to his exhibition of Surirella gemma, with Zeiss's 
D (g-th) lens at the recent Scientific Evening of the Royal Microsco- 
pical Society, in confirmation of his views. I refrained from com- 
menting on that exhibition, thinking, after his apology for its failure 
on the evening, the least said about it the better. But, since he again 
insists on referring to the exhibition, I venture to inform those of your 
readers who did not witness it, that, compared with Dr. Woodward’s 
photograph of Surirella gemma (which may be accepted as the finest 
definition hitherto obtained of it), Mr. Slack’s image was only recog- 
nizable by the general contours — not by the superficial definition. It 
was the old story — Amplification versus Definition — and he seems to 
prefer the former. 
He says Zeiss’s lenses have taught him a lesson on the utility of 
small apertures. The information he seems to have received from their 
examination has been commonly accessible for at least forty years past : 
for example: — In vol. ii. of the Natural Philosophy published in 
1832 by the Society for the Diffusion of Useful Knowledge, p. 50 of 
the Treatise on Optical Instruments by Sir David Brewster, we find 
the lesson summarized : 
“ It should be remarked, that when the objects arc used as opaque, 
