212 
to say, whether in parietal bands or radiating central masses, and 
the only question, indeed, was whether such could be rightly 
regarded as characteristic of groups or as more special to indi- 
‘vidual forms, that is (more broadly) of generic value, or (more 
restrictedly) of simply specific importance. 
Mr. Archer showed a new form referable to the genus Oocystis 
(Al. Braun.). This was minute, elliptic, and presented the pecu- 
liarity of being clothed with a number of elongate, very slender 
and delicate hair-like processes, quite as long as the longest dia- 
meter of the cells. It would require a figure, however, to convey 
a due conception of objects of such minute size, and further to 
dilate upon the present form here might not prove of much use. 
8th December, 1870. 
Dr. John Barker showed a Saprolegnian obtained from Am- 
blyopsis speleus, showing sporangia and zoospores ; no oogonia 
were evinced, hence the species not determinable; the plant was 
most likely Achlya prolifera, obtained, however, from probably a 
new host. 
Mr. Archer exhibited some fine active living examples of Vasicola 
ciliata (Tatem) and Anthophysa Miilleri (Cohn), perhaps to some 
extent of interest, taken in thus almost midwinter. 
The following communication was made to the Club :— 
On an Achromatic Combination for use with Blue Light. 
By G. Jonnsrone Stoney, M.A., F.RB.S., &e. 
In achromatic combinations of flint and crown glass only two 
points of the spectrum can be accurately brought to focus toge- 
ther. With lenses as usually achromatised the green rays are 
those brought to focus nearest to the lens, then the red and blue 
a little farther out, and, lastly, at a considerable interval, the violet. 
Such a lens will give for the image of a white point a small patch 
of light consisting of red, yellow, green, and blue, surrounded 
by a haze of indigo and violet, which, from being faint, does little 
harm when the lens is used in the ordinary way. 
But if we wish to see the most minute objects with the micro- 
scope, there would appear to be an advantage, as was pointed out 
to the Club in acommunication made on the 21st of March, 1867,} 
in using only the blue and violet rays; and in this case an achro- 
matic lens of the ordinary construction will plainly not produce 
the best effect. Better definition would be obtained by using a 
lens which is what the opticians call under-corrected, so ad- 
justed that the indigo rays shall be those first brought to focus. 
Such a lens will unite the indigo, blue, and violet rays emitted 
by a point of the object, into a very small patch. With white 
light this would be surrounded by a haze of the red, yellow, and 
yellowish-green rays; and these, being the brightest rays of the 
spectrum, would blur the image and destroy all accurate defini- 
tion. 
1 See ‘ Microscopical Journal,’ vol. vii, n. s., p. 234. 
