230 Transactions of the Royal Microscopical Society. 
Now let the test object be Pleurosigma formosum. It will be 
found quite impossible to get a distinct view of the beads with any 
considerable power, say 3000 to 5000 diameter or upwards, unless 
the colour corrections of the lens are such that the beads appear of 
brilliant red on a greenish ground, that is, unless the objective is 
considerably under-corrected. When this is the case good photo- 
graphs can be made after the interpolation of an ammonio-sulphate 
cell, but when the glass is more nearly achromatic not merely the 
photographs are unsatisfactory, but with white light it will be 
found impossible to separate the beads distinctly on the screen. 
I do not wish to lengthen unnecessarily this already long 
article ; but so much has been said of late with regard to “ the 
advancing powers of microscopic definition ” that a few further facts 
and considerations appear to be demanded. 
I note, first, that certain Continental makers would appear to 
have worked out the problem of high-power definition to about the 
conclusions indicated above. I have had recently in my hands a 
new No. 10 immersion by Hartnack, the property of Dr. A. Litton, 
of St. Louis, Missouri, which handsomely resolved the 19th band, 
and gave on Pleurosigma formosum the colour indications com- 
mended above. This objective measured at fifty inches 700 dia- 
meters uncovered, 770 covered. 
I have, moreover, recently examined a new Gundlach No. 8, the 
property of Mr. W. H. Walmsley, of Philadelphia, which also 
resolved the 19th band in a satisfactory manner. This objective 
measured at twelve inches distance 200 uncovered, 220 covered. 
Tested on Pleurosigma formosum the colour corrections were 
almost identical with those of the Hartnack objective. 
I may add to the above, that during the past six months, four 
objectives by Powell and Lealand marked xx th have passed through 
my hands. Three of these were at the time the property of a well- 
known importer of optical apparatus, the fourth had been made to 
order for a distinguished microscopist in New York. All these 
glasses compared favourably with the Museum x Yth, though there 
were some small differences in magnifying power. All had the 
same colour corrections. I saw, however, in none of them any 
superiority over the objective furnished me in 1869. 
It seems probable to me therefore that the distinguished makers 
last named have made no substantial progress since 1869, and this 
view is confirmed by Dr. Pioyston-Pigott, who mentions in this 
Journal, February, 1872 (p. 66), that Powell and Lealand had 
placed at his disposal “ for a fortnight ” a x Vth “ similar, though 
perhaps slightly superior, to the celebrated immersion X(r th signalized 
by Dr. Woodward,” and which he says the makers were “unwilling 
to dispose of,” on account of its excellence. 
It would appear therefore that the recent advancing powers of 
