NOTES AND CORRESPONDENCE. 
Count Francisco Castracane’s New Method of Ilumination.— 
Will you allow me to offer a few remarks on the letter refer- 
ring to a new mode of illumination by Count F. Castracane, 
which appears in the last number of the ‘ Quart. Journ. Mic. 
Sci.,’ especially with reference to the diatoms on which it is 
desirable to test the powers of the new method of illumina- 
tion. It is perfectly true that a few years ago Pleurosigma 
angulatum was recommended as a valuable test object for 
good objectives; but it, ike the scales of Podura, is now 
scarcely recognised as a test for the best 4-inch and higher 
objectives of large angular aperture. You very properly 
observe in your foot-note P. angulatum is now recognised as 
very easy of resolution. I beg to suggest to Count F. Castra- 
cane the desirableness of his trying experiments with all or 
any of the following diatoms, the striz on which are much 
more faint and close than are those of P. angulatum, and the 
proper resolution of which is a severe test, not to tths and 
ths only, but to objectives of even higher power. 
If the monochromatic mode of illumination suggested by 
Count F. Castracane enables him to resolve with moderate 
distinctness the markings on Pleurosigma arcuatum, Donkinia 
carinatum, D. rectum, D. minutum, its adoption would be of 
great service to all microscopists who are interested in the 
study of Diatomacez. 
I have endeavoured, but unsuccessfully, to resolve the 
markings of the diatoms just enumerated, and have used an 
excellent 1th with Nos. 1 and 3 eye-pieces and draw-tube. 
Pleurosigma angulatum, Foxonidea insignis, Pleurosigma lan- 
ceolatum, and other finely marked diatoms, are easily resolved 
by the appliances I have at command; but the other forms 
named baffle all my endeavours to resolve them. 
I shall be most happy to supply Count F. Castracane with 
specimens of all the Diatomacez I have named, gathered from 
the Northumberland coast, either mounted ready for inspec- 
tion or prepared in readiness for being mounted. No address 
