32 
CORRESPONDENCE. 
Powell and Lealand's gth new Immersion Objective. — The 
' Academy ' (May 8) says of this glass : " We have had an opportunity 
of trying one made for Mr. Lettsom, and it is certainly a very 
remarkable production, able to show very minute structures for which 
much higher objectives have hitherto been employed. It has a con- 
siderable working distance in proportion to the magnification it 
affords with deep eye-pieces, and gives a wonderful view of diatoms 
flat enough for its angle of aperture and contiguity to the object. It 
has also sufficient penetration for small live objects, and has plenty of 
light with D and E eye-pieces, which cause no noticeable deterioration 
of its performance when, as should always be the case with high 
powers, an achromatic condenser is employed." 
COEKESPONDENCE. 
. W HAT ARE THE CHARACTERISTICS OF FrUSTULIA SaXONICA ? 
To the Editor of the ^Monthly Microscopical JournaV 
Denstone, May 18, 1875. 
SiE,— I am afraid the 'Monthly Microscopical Journal' (vol. ix., 
p. 86) must have made some mistake, and have misrepresented Dt. 
Woodward in that paragraph headed " Feustulia Saxonica as a 
Definition Test." What is there said is by no means very clear ; 
but it certainly does make him assert one of two things : either (1) that 
FrustuUa Saxonica is a one-lined object (i. e. has transverse, but no 
longitudinal lines); or (2) that, though it undoubtedly has transverse, 
and may possibly have longitudinal lines as well, no one as yet has 
succeeded in seeing the latter, but that those who fancied they saw 
them, as Dippel and others, have been deceived by " diffraction 
phenomena." 
If either of these interpretations represents his real views, I would 
request Dr. Woodward of his patience and courtesy to permit me to 
make a few remarks. 
In the summer of 1872 I called upon Herr Seibert at Charlotten- 
burg for the purpose of procuring one of his high-power immersion 
lenses. On this occasion, after letting me see what his manipulative 
skill could accomplish in resolving the tests I had brought with me — 
and amongst the first was an extremely fine-lined specimen of FrustuUa 
Saxonica — he next showed me a couple of very beautiful photographs 
of that diatom, one of which exhibited the transverse, and the other 
the longitudinal lines, with far more clearness, sharpness, and dis- 
tinctness than the printer will be able to reproduce the words I have 
here written. 
Some ten days later I called upon him again between eight and 
nine in the evening, and before he would let me go he insisted on 
Bhowing me what the glass I had selected could do on his own private 
