APPARATUS FOR HOLDING SPECIMENS 63 



cover glass sliould be smeared round its edge with olive oil 

 or vaseline. 



A centrifuge is a small hand macMne for revolving test tubes Oentniuge. 

 of fluid at a very rapid speed, so that the heavy portions of 

 sediment may be rapidly separated from the fluid. Two glass 

 test tubes encased in aluminium covers are revolved at a speed 

 of about 2,500 revolutions per minute by turning a handle. 

 The examination of urine is greatly facilitated by this method, 

 and hyaline cysts can be deposited without breaking them or 

 altering their form. Milk is separated by the centrifuge so as to 

 give the percentage of fat, and micro-organisms can be readily 

 concentrated to the bottom of the test tube, from which they may 

 be extracted with a pipette. 



A simple microspectroscope for the examination of blood has Micro- 

 been designed by Mr. Rheinberg ; it consists of the micrometer 3p«'='"'S'=°p*' 

 eyepiece, as described on page 68, with a sUt in the position 

 where the divided glass plate is generally placed, and a difErac- 

 tion grating placed in the eyepiece. On looking through the eye- 

 piece, the slit is observed, while to one side a spectrum is formed. 

 If a low-power object!glassbeused,and the object to be examined 

 placed on the stage of the microscope, its spectrum wiU be seen 

 some little distance to one side of the slit. If a comparison 

 slide of a fluid be prepared close to the edge of a glass slip, 

 it can be placed on the stage in contact with the fluid to be 

 examined on the edge of another slip, and the two spectra can 

 be seen at the same time one above the other. If colour filters 

 are to be examined they can also be compared by this 

 method. It is very useful for the examination of blood, chloro- 

 phyll, dyes, or other colouring matter. 



The preparation of metallurgical specimens for examination Metai- 

 imder the microscope consists of cutting off a small piece of the ^l^fi^ina. 

 metal to be examined with a hacksaw and grinding a small 

 portion to a flat surface and polishing it. It is then etched with 

 such solution as will remove certain constituents from the 

 surface, leaving the rest unaffected. Where a fracture of steel 

 is to be examined, it is sometimes advantageous to cover it, before 

 grinding and polishing, with a coating of copper by electro- 

 plating, as by this means a fractured edge shows 'up very clearly 

 against the different colour of the copper. 



The piece so polished is then mounted by embedding it in a Mounting 

 lump of wax placed on the slide. The best wax for this purpose ^thwas! 

 is one prepared in such a manner that it will hold its position 

 for a long period and yet remain plastic under pressure. It is 

 known as S.I.R.A. wax. The specimen should be attached so 

 that its surface is parallel to the slip upon which it is mounted, 

 and this is done most readily as foEows : 



Cut two square or circular pieces of wood or vulcanite from 

 the same piece of material of a thickness greater than the specimen 



