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Thirteenth Report of 
alternative of having portions of them well delineated while 
the rest is indistinct. Other difficulties attending Micro- 
scopical Photography have been pointed out in Mr. Hodgson's 
paper, on which it is not necessary for me to dilate. 
In spite of these obstacles, I venture to prophesy that this 
beautiful art will flourish; for its want of universal appli- 
cability need not prevent its use in the numerous cases to 
which it is appropriate. 
The paper of the Rev. William Smith on the Stellate 
Bodies occurring in the cells of fresh-water Algae gives some 
further details of these plants, which may hereafter assist us 
in forming a more correct theory of their physiology. 
Our Secretary, to whom we have formerly been so much 
indebted for valuable contributions, has recently read a very 
interesting account of some observations he has made on the 
presence of a fungus, and of masses of crystalline matter, in 
the interior of a living oak tree ; a circumstance which does 
not appear to have been previously noticed, and which hardly 
admits of a satisfactory explanation in the present state of our 
knowledge. It is not, however, less worthy of record on that 
account ; for all sound theory must be based upon carefully- 
observed facts ; and the first fact of a kind is at least as 
valuable as those which may hereafter follow it. 
The very large demand for first-class microscopes, which 
has increased rather than diminished during the past year, has 
stimulated the makers to use every exertion to extend to the 
utmost the apertures of their object-glasses. Messrs. Smith 
and Beck have produced a 4-lOth inch of upwards of 90°, 
chiefly valuable for the examination of opaque objects. Mr. 
Ross has lately made some objectives of l-8th inch focal 
length, and 155^ of aperture, which, by permitting very 
oblique illumination, bring out the markings on the most 
difficult test objects in a highly satisfactory manner. Mr. 
Wenham, in following up his experiments to ascertain the 
limits of useful aperture, has constructed a glass of 170°, and 
l-12th inch focus ; but is still of opinion that nothing is gained 
beyond 150°. From a very brief examination of his object- 
glasses, I am inclined to differ with him, and to think that for 
the purpose of merely discovering the existence of very close 
lines or dots the aperture cannot be too great. For the useful 
application of the microscope to minute anatomy and physi- 
ology a much smaller aperture will suffice, which, from not 
requiring such careful adjustment, and such close proximity to 
the object, is far more convenient in use. 
Wiien we consider that the real aperture of an object-glass 
is the chord of the angle at which light is admitted, and that the 
