258 THE MICROSCOPICAL NEWS, 
tion between aperture and focal length ; and that this proportion 
may be determined with such a degree of approximation as is 
required for a practical guide, at all events sufficient for showing 
the limits between rational and irrational aims. 
In my opinion the question discussed here is of some general 
importance in regard to microscopy. It will, of course, do no 
harm that systems of lenses should be made of any design whatever 
and according to any particular taste, and full liberty must always 
be conceded in that respect. On the other hand, however, the 
Microscope has an important vocation as an aid to scientific 
research, and microscopical science is therefore fully justified in 
demanding that the prominent feature in the improvement of the 
instrument should always be to render it as useful as possible for 
its primary purpose, and that no hobbies of any kind should be 
permitted to take the lead in microscopical optics. In order to 
prevent this, and to secure progress in the direction of useful aims, 
the discussion of the question of the “rational” construction of 
objectives cannot be dispensed with. 
(Finis.) 
SOME EXPERIMENTS ON THE PHYSICS AND 
CHEMISTRY OF THE SAP OF PLANTS. 
By PROFESSOR JOHN ATTFIELD, Ph.D., F.R.S., F.C.S., Etc. 
Read at Watford, 17th April, 1883. 
i. plants grow, whence comes their food, what is its nature, 
and how it is conveyed, are questions which have interested 
man in all ages. And to earnest, intelligent, unprejudiced truth- 
seeking enquirers answers to the questions have been obtained ; 
answers general and incomplete at first, not even yet minute and 
perfect. 
Burn a log of wood in a grate with due access of air. The wood 
and the oxygen of the air together yield carbonic-acid gas, vapour 
of water, and nitrogenous gas, all of which pass up the chimney ; 
ash containing various mineral matters, which falls beneath the 
grate ; and heat, which warms and comforts us. Conversely nature 
brings together heat, mineral matters, nitrogenous gas, water, and 
carbonic acid gas, and the products are wood and oxygen. To 
extract temporary comfort from the wood of our trees (or from coal, 
which is only altered wood), we convert that wood into warmth, 
smoke, and ash, and at the same time use up life-giving air. Bene- 
