151 
NOTES AND MEMORANDA. 
mounted in balsam. The highest eye-pieces made may be used 
without any practical detriment to the image, although, of course, 
with a reduced sharpness of the definition. 
“ On the whole, I think it in many cases the finest lens, of its 
power, that I have ever seen ; and in every sense it is an admirable 
acquisition.” 
In conclusion, Mr. Dallinger refers to “ another feature in the use 
of this lens which is a drawback,” viz. that the oil is a solvent of 
most varnishes and gums used in mounting and finishing slides 
(which may be remedied by coating the edge of the cover with 
shellac varnish) ; and also to the necessity for the objects, such as 
frustules of diatoms, to be “ burnt ” on to the cover or mounted in 
balsam or other fluid with an equal refractive index. 
Dry V. Immersion Objectives. — In the same letter Mr. Dallinger 
observes : — “ Even water immersion lenses are of very limited service 
in observations continuously conducted upon minute living organisms 
in fluid. We may gladly call in their aid, in the determination of a 
delicate change of form, or in the more perfect detection and defi- 
nition, of an obscure point of structure ; but for steady and constant 
work we are bound to avoid them ; for the fluid under the delicate 
cover is in danger every moment of being ‘ flooded ’ by coming into 
contact with the water on the top of the cover, and between it and the 
lens ; because the movements of the organism have to be counteracted 
by the movements of the mechanical stage, in order to keep any form 
that may be studied in view constantly. But this opens to us the 
possibility of going to the edge of the cover at any moment ; and thus, 
by the mingling of the fluids, rendering the observation void. This, 
of conrse, will apply still more fully when, as in the case of the valu- 
able glass of Zeiss, the ‘ immersion fluid ’ is an essential oil. 
“ Happily it is only in special cases that the greater analyzing 
power, combined with larger working distance, which is possessed by 
immersion lenses, is required. It is in the earlier study of an 
organism, and before continuous work upon it has begun. And even 
if it be not, in the majority of cases a first-class dry English lens of 
a higher magnifying power, if efiiciently used, accomplishes all that 
is required. Hence the fine ‘ new formula ’ lenses, dry (also provided 
with fronts to be used as immersion lenses), are as yet an unsurpassed 
boon for this special class of work. And certainly it is one which, in 
relation to biology, has a most important future. I know, of course, 
that the optician has irresistible limitations to deal with ; but the 
‘ new formula ’ dry lenses I have referred to, prove, in comparison 
with the preceding lenses, made by the same firm, that the dry lens 
was capable of most serviceable improvement. What is important, 
therefore, is that the larger demand for lenses that will resolve readily, 
difficult lined and beaded objects, which can certainly be best done, 
all things being equal, with immersion lenses — and to the improved 
manufacture of which Zeiss’ oil immersion gives aj)parently a new 
departure — should not lead the best opticians in England, the Conti- 
nent, and America to abandon efibrts for the still greater improve- 
ment of their dry lenses. They are of the greatest value to the 
practical biologist, working amidst the minutest living things in 
nature, and from the study of which so much may be anticipated.” 
