136 



HARDWICKE'S SCIENCE-GOSSIP. 



place the slides on the top, and on each a flat bullet 

 or large shot. I then put a gaslight under the 

 box, and keep the water nearly boiling for about 

 forty-eight hours : the slides will then be sufficiently 

 baked, and may be cleaned, and finished off by 

 putting a ring of black, or other varnish, round the 

 edge of the glass circle. I found the use of the 

 spring clips very objectionable, as I was always 

 getting too much or too little pressure on my 

 slides ; but by having various sizes of bullets and 

 shot, I can put just the weight I require on each 

 slide. The bullets are flattened by striking them 

 with a hammer. This process prevents all possi- 

 bility of getting the balsam to the boiling-point, 

 and at the same time gives as much heat as is 

 required. I have exchanged many slides with your 

 correspondents, who have, without exception, ex- 

 pressed a very favourable opinion of them. I 

 should say that, if the object is very thick, and 

 consequently the balsam thick round it, it should 

 bake rather longer. The two funnels are merely to 

 allow of the expansion and contraction of the water 

 as the heat varies. My objections to Mr. Kitten's 

 plan (though in his hands no doubt through practice 

 it is successful) are that air, if it gets in, will bo 

 difficult to get out, instead of going Out itself, as in 

 my plan ; — that it affords no opportunity of care- 

 fully examining the object when on the glass to 

 remove any dust or hairs ; — that the wire clip will 

 often crush a valuable object, or not subject a 

 strong object to sufficient pressure, and that the 

 slide will sometimes get too hot over the lamp and 

 spoil all the work. I also use chloroform in other 

 ways, but I have trespassed sufficiently upon your 

 space at present. — C. L. Jackson. 



A PoLAKiscoPE.— " F. M. S." will find a descrip- 

 tion in "Pereira's Lectures on Polarized Light," of 

 an apparatus constructed by^Prof. Powell for examin- 

 ing the polarization of light by fluids. There is 

 also a fuller account of the apparatus in the Philo- 

 sopJiieal Magazine, April, 1S43. The amount of 

 rotation which a ray of light suffers during its 

 passage through the liquid is measured by an index 

 attached to the analyzer, and moving on a graduated 

 circular metallic plate. — R. H. N. B. 



"Inteefekence."— In reply to Horace 'Wilson's 

 question, I beg to say that if he will light his lamp 

 when the daylight is failing, and hold a pencil 

 perpendicularly on a sheet of paper near the win- 

 dow, so as to balance the lamp- and day-light, 

 he will see two shadows of the pencil on the 

 paper. One, cast by the lamp, will be illuminated 

 by the blue daylight, and (surrounded by the 

 yellow lamplight) will appear blue, as "H. W." de- 

 scribes. The other, cast by the daylight and 

 lighted by the lamp, he will see to be a bright 

 yellow, being surrounded by the yellow lamplight. 



The experiment shows, by strong contrast, the 

 great difference in colour between artificial and sun- 

 light. It has nothing to do with interference, but 

 is interesting to microscopists, artists, &c., who 

 work by both lights. — G. W. 



Definition of Stuuctukes.— So far as I under- 

 stand Mr. Thomas's query, I am afraid he requires 

 an impossibility, for it is most assuredly impossible 

 to obtain a high amplification, and at the same to 

 have a large portion of the object under examina- 

 tion visible ; a 2-inch objective shows about a i of 

 an inch, a 1-inch about ~ of an inch, and a ^-inch 

 -^ of an inch of surface. Surely there would be 

 no difficulty ''in discovering and clearly and dis- 

 tinctly defining what is required, viz., the form, 

 position, &c., of the object under examination," by 

 successively increasing the magnifying power, com- 

 mencing with a 3-inch, and afterwards using a 

 1-inch and i-inch objective, particularly if a bin- 

 ocular is employed. — K. 



The " Science-Gossip " Section-cutter. — In 

 No. 92 (August 1, 1S72), an instrument for cutting 

 vegetable sections was figured and described by 

 Mr. W. White, of Litcham. The instrument there 

 described was a copy of the one he was using 

 himself; he has since made some improvements in 

 it, and at the request of several friends, he has had 

 some manufactured under his own directions, and 

 those interested in the preparation of sections of 

 vegetable structures will do well to procure one of 

 them. We have tried this form of section-cutter, 

 and find it much more manageable than the old- 

 fashioned one with the screw. The great secret of 

 the success of the present instrument is the eleva- 

 tion of the material to be cut by percussion. This at 

 once overcomes the "drag" upon the cork, which 

 the screw often fails to do until it has been turned 

 more thau is necessary, and we get a section either 

 too thick or unequal. The sections cut by Mr. White 

 are certainly far superior to any we have seen before- 

 A further advantage which most microscopists will 

 appreciate is that the price is not above a third of 

 that usually charged for the screw instrument. With 

 the instrument he sends a few practical directions 

 for its use.- — F. K. 



Sacchako-Polakiscope. — If F. Kitton had con- 

 structed and tried his " Saccharo-Polariscope " 

 (figs. C2 and 63, page 104, in your May number), 

 he would have found he had made a very important 

 mistake in using a tube with a round bottom or 

 test-tube form, which, acting as a plano-convex lens, 

 would prevent any light reaching the eye. This of 

 course is seen at a glance by any one at all ac- 

 quainted with optics, but not so readily by young 

 people who very laudably try to make apparatus for 

 themselves. I need scarcely say the bottom of the 

 tube should be ground off, and a disc of thin " patent 



