236 



SCIENCE-GOSSIP. 



MICROSCOPY 



CONDUCTED BY J. H. COOKE, F.L.S., F.G.S. 



To whom Notes, Articles and material relating to Microscopy, 



and intended for Science-Gossip, are, in the first instance, to 



be sent, addressed "J. H. Cooke, Thorndalc, Lincoln." 



The Rings and Brushes of Crystals. — Those 

 who are possessed of a polariscope as an accessory 

 to their microscope might as well do a little physics, 

 and employ it in the observation of that beautiful 

 and interesting optical phenomenon known as the 

 rings with a cross, or the rings and brushes of 

 crystals. Some treatises on the mounting of micro- 

 scopic objects give directions for the cutting of 

 these crystal sections ; but the methods of observing 

 them under the microscope are seldom described 

 and, I believe, are rarely known, at least among 

 amateurs. The crystals most commonly employed 

 are calcite, Rochelle salt, borax, phosphate of soda, 

 nitre, ferrocyanide of potassium and bichromate of 

 potassium, all of which, except the two last, require 

 to be specially prepared by cutting sections perpen- 

 dicular to one of their optic axes. Practically it 

 is not always easy to find where the optic axis 

 exactly lies, but I have found by experience that 

 for borax a longitudinal section, for phosphate of 

 soda a transverse, and for Rochelle salt an oblique 

 section, of the respective crystals will furnish the 

 object desired. The sections, taken about a quarter of 

 an inch thick, are smoothed to about one-eighth of an 

 inch thick with water on a hone, taking care that the 

 flat sides are kept strictly parallel. The preparation 

 of calcite requires special manipulation on account 

 of its splintery cleavage ; a piece must be slowly 

 cut off by a fine saw (made by fastening a piece of 

 steel ribbon in a groove cut in a wooden stick) at 

 right angles to the long axis of the hexagonal 

 prism, and then cautiously smoothed flat and 

 rather thin on a hone moistened with paraffin. 

 Some selected crystals, on the other hand, such as 

 those of sugar, sulphates of zinc and of magnesia, 

 may be examined without any preparation. All 

 sections as soon as made must be immediately 

 mounted in watchmaker's oil in a glass or tin cell 

 of the requisite depth, and closed with Miller's 

 cement, or best colophony-shellac varnish. Having 

 manufactured as excellent a section as you can 

 possibly manage, i.e., one as nearly vertical to the 

 optic axis as can be hit upon, the next design and 

 endeavour will be to apply the micro-polariscope 

 so as to discover and exhibit its double refraction 

 phenomena efficiently and conveniently. For this 

 purpose it is imperative that the crystal section be 

 examined under convergent polarized light, and 

 that the phenomenon thuswise revealed, and not 

 the image thereof, should be viewed directly by 

 the objective. Now in order that the light 

 polarised by the nicol below the stage be rendered 

 convergent, it is necessary to surmount it by a 

 double-lensed condenser. An achromatic condenser 

 of short focus may be employed, but I have found 

 a doublet composed of a hemispherical lens of five- 

 eighths inch diameter, with a similar lens of five- 

 eighths inch diameter placed above it, to be very 



effective and quite sufficient. Proceed now as 

 follows : Place the slide on the stage, and push up 

 this condenser as near as possible to it. Use a 

 quarter-inch objective below the analyser on the 

 nose-piece, and put the nicols in the crossed 

 position, i.e., where there is " no light, but rather 

 darkness, visible." Lower the objective close 

 down so as to touch the cover-glass of the slide, or 

 nearly so, and then remove the eye-piece. The 

 rings and cross of a uniaxial crystal, or the rings 

 and brushes of a biaxial crystal will then be 

 observed with illustrious beauty, and a clearness 

 corresponding to their size and the perfection of 

 their condition. There is a considerable advantage 

 in using a large polarizer, as thereby the field of 

 view is enlarged, and the same effect is produced 

 by having the condenser as strongly convergent as 

 possible. By all means place a biconvex lens of 

 about three inches focus immediately above the 

 analyser on the nose-piece, so as to enlarge the 

 image and render it more distinctly visible. — 

 Dr. P. Q. Keegan, Patterdale, near Penrith. 



Mounting Volvox. — The best months for 

 obtaining Volvox or Desmids are July and August. 

 Place the gathering in a small tank or a glass 

 tumbler and stand it in a well-lighted window for 

 a few days. The volvoces will collect on the side 

 nearest the light, and maybe readily taken out with 

 a pipette. Place them in a watch-glass contain- 

 ing alcohol, three ; distilled water, two ; glycerine 

 (sp. gr. i'25o), one. Allow this to stand for a week 

 or two under a glass tumbler, at the end of which 

 period the water and spirit will have evaporated 

 and the algae will be left in pure glycerine. Mount 

 in a cell and ring with asphalte. 



Odontophores of Snails. — These serve as 

 excellent objects for the polariscope. To mount 

 them use a weak form of Goadby's solution, and 

 apply just sufficient pressure to open out the 

 ribbon, but not enough to reduce everything to a 

 dead level. 



Refractive Indices of Certain Liquids. — The 

 following table of refractive indices, taken from a 

 number of the " Scientific American," may be of 

 some value to those of our readers who are experi- 

 menting with liquid mediums : water, i'336 ; 

 alcohol, i'372; muriatic acid, i'4io ; nitric acid, 

 1-410; sulphuric acid, 1434; olive oil, i'47o; oil 

 of turpentine, i'475; castor oil, i'49o; balsam, 

 copivi, 1528; Canada-balsam, 1549; oil of cloves, 

 i - 535 ; oil of cassia, 1-641 ; sulphuret of carbon, 

 1768. 



Dead-Black Varnish. — Methylated spirit, five 

 ounces ; orange shellac, a quarter ounce ; lamp 

 black, half ounce. Dissolve the shellac in the 

 spirit, then rub the lamp black very fine and add 

 the liquid gradually. 



A Fly's Foot. — The walking of a fly on the 

 ceiling is a familiar phenomenon not yet fully 

 understood. A recent paper by Mr. D. H. Dierhold 

 mentions that the microscope quickly disproves 

 the old theory that flies hold to smooth surfaces by 

 means of suckers, and that Hooke's idea that flies 

 stick to glass by a viscous secretion was shown a 

 dozen years ago to be only partly sound. Dr. 

 Rombout has established the fact that the flies 

 hang on by the help of capillary adhesion — the 

 molecular attraction between solid and liquid 

 bodies. It is true the foot hairs are very minute, 

 but, as each fly is said to have 10,000 or 12,000, 

 we need not be surprised at what they can do. 



