213 
But the case would be very different if these rays were ex- 
cluded ; as, for example, by employing direct sunshine or other 
intense source of light, and interposing a sufficient screen of the 
ammonio-sulphate of copper. Under these circumstances, a lens 
of the proposed kind may be expected to perform in a very marked 
degree better than one of the usual construction. But to ex- 
hibit its advantage, it is essential that all rays of lower refrangi- 
bility be stopped. This can be easily effected by ammonio- 
sulphate of copper, but is not effected by the cobalt-blue glasses 
which are commonly used. These shades, when dark, have the 
double disadvantage both of weakening the blue light in a consider- 
able degree and of allowing the extreme red to pass abundantly. 
With object-lenses which have a flint-glass lens between the 
convex of crown glass and the image, the requisite adjustment 
might be attained by separating these lenses; and in all cases it 
would appear to be possible to effect it very simply by interposing 
a shallow plano-convex of crown glass between the object-glass 
and the image, taking care to turn the convex side towards the 
object-glass, so as not to increase the spherical aberration. By 
shifting this lens backwards and forwards it would be easy to 
determine the position in which it will give the best definition 
with blue light. 
Rey. E. O’Meara exhibited a slide containing several diatoma- 
ceous forms from mud furnished by Professor Sullivan. The 
locality in which the material was gathered was not described 
more particularly than that it was on the coast of the Co. Clare. 
The forms contained, however, indicated that it was near the 
shore in the vicinity of a stream or river which proceeded from 
neighbouring hiils. 
The following is a list of the forms :-— 
Actinoptychus senarius. Pinnularia distans. — 
“A triradiatus. os major. 
Achnanthes brevipes. cc mesolepta. 
Cocconeis scutellum. 5 peregrina. 
“e - var. B. 35 divaricata. 
Coscinodiseus cervinus. Pleurosigma balticum. 
A excentricus. si fasciola. 
Diatoma grande. 55 formosum. 
Grammatophora marina. a. quadratum. 
“ serpentina. fe Wansbeckii. 
+5 macilenta. Podosira Montagnei. 
Melosira Westii. 5 maculata. 
Navicula cluthensis. a3 hormoides. 
o punctulata, Raphoneis rhombus, Hhren. 
% pygmea. i amphiceros, Lhren. 
a solaris. Stauroneis salina. 
” elegans, Schizonema cruciger. 
Nitzschia sigma. Synedra affinis. 
¥ panduriformis. r pulchella. 
Orthosira marina. Scoliopleura convexa. 
Ps angulata. a Jenneri. 
- physoplea = Smithii. 
Discoplza physoplea? Ehren. 
2) 
Tryblionella punctata. 
is acuminata. 
