442 General Notes. [July, 
go and destroy as many villages, and catch as many people as he could 
for slaves. The Warua, especially, although holders of slaves, would 
rather die than be slaves themselves. I have heard instances of their 
being taken even as far as the Island of Zanzibar, and then making their 
way back, single-handed, to their own country. The only thing that 
will do away with slavery is opening up Africa to legitimate commerce, 
and this can be best done by utilizing the magnificent water-systems of 
the rivers of the interior. 
MICROSCOPY .! 
WYTHE’S ILLUMINATOR. — Dr. J. H. Wythe recommends for oblique 
illumination a right-angled prism with a plano-convex lens, cemented to 
and coyering one of its narrow sides, and an ordinary French triplet 
fastened to the other, close to the farthest angle. Arranged with the 
plano-convex lens directly downward, the axis of the triplet would be 
horizontal, and a horizontal cone of achromatic light would be furnished ; 
while by slightly tilting the apparatus an available and extremely 
oblique illumination is obtained. 
Saw Francisco Socrery. — At the annual reception of this society, 
twenty-two members exhibited a large number of objects from the min- 
eral, vegetable, and animal kingdoms. The intelligent classification of 
the views was a notable improvement upon the management of too 
many exhibitions of this kind. 
Biiven’s Pnorocraprns. — Mr. R. H. Bliven, of Elmore, Ohio, is 
now supplying good photographs of a large variety of objects. He also 
makes to order photographs of any suitable slide. Such pictures of 
familiar objects are very interesting. They are doubly important if the 
slides are particularly choice or rare, as a partial protection in case of 
accident to the objects themselves; while for educational purposes they 
are often available under circumstances where a resort to the microscope 
itself would cause too much interruption or delay. 
APERTURE OF AN OBJECTIVE. — [ Mr. Tolles contributes the follow- 
ing note in regard to the aperture of an objective marked 180°, which 
was sent to London some years ago, and has been the object of no little 
discussion ever since, : 
The diameter of the exposed front surface of an immersion objective, 
is given as .043!’, the point of focus as obtained by using only the ray" 
emerging from the front, comparatively near to the axis, = 013", and ® 
clusive against any more than 1 
air-angle in the objective. 
jective was marked 180° 
A year afterwards the author of rs 
Fra. 25. diagram, Mr. Wenham, commie a, 
another item relating to the angular aperture of the same lens. He giv 
1 This department is conducted by Dr. R. H. Warp, Troy, N. $ 
