Practical Hints. 53 
angular aperture, or, should it be too wide, limiting this otherwise 
excellent quality by a diaphragm cap being slipped over the end. 
Another way by which stereoscopic photo-micrographs can be 
obtained by a monocular microscope is to employ an objective 
having an effectively large front lens, and covering it with an easy- 
fitting cap, having in it an aperture so much at one side as to 
cover up one half of the lens. When making the first exposure, 
the cap is turned so as to uncover one side of the lens, and is 
rotated half a turn before taking the second negative. The 
resulting pair of pictures will be stereoscopic. 
There are several other methods which may be employed, and 
which are more especially adapted for the higher powers. This 
article is, however, mainly intended for the photo-microscopic 
aspirant with limited appliances. 
Practical Hints. 
Flexible Mucilage.—To 20 parts of alcohol add 1 part of 
salicylic acid, 3 parts of soft soap, and 3 parts of glycerine. Shake 
well, and then add a mucilage made of 93 parts of gum arabic 
and 180 parts of water. ‘This is said to keep well and to be 
thoroughly elastic. 
Spores of Equisetacee.— An interesting microscopic amusement 
for this season of the year, is to gather a few heads of Equisetaceze 
(horsetails), which will soon be readily obtained. If some of the 
spores be taken on a glass slip, placed upon the stage of the 
microscope, with a r-inch objective, and gently breathed upon, 
the effect is magical. Each of these spores has two pairs of 
elastic filaments, which, when exposed to warm moisture, jump 
and curl in the most amusing manner, the object of thi, 
movement being the same as that of the fern antherozoids—viz. 
the fertilisation of the embryonal corpuscle. 
Good Polariscope Object.—A section of oak bark, carefully cut 
in a direction parallel with the outer surface, and down the tree, 
mounted in balsam after prolonged soaking in turpentine, forms a 
very lovely polariscope object, with an inch objective, and no 
selenite. Long linear series of cells, each containing a crystalline 
body, are seen as many-coloured lines of stars, when the Nicol’s 
prisms are crossed, 
