1883.] 



MICROSCOPICAL JOURNAL. 



117 



the centre, and heat is applied until 

 the balsam becomes hard when cool. 

 The excess of balsam is then re- 

 moved with an old pocket-knife, and 

 the slide finally cleaned with am- 

 monia or alcohol. 



In the above process we have 

 departed from the usual course, by 

 using carbolic acid. It will be found 

 on very short experience, that car- 

 bolic acid is far superior to alcohol 

 for mounting purposes. The acid is 

 used in its strongest fluid condition, 

 and it then freely mixes with balsam. 



As thus prepared, the compound 

 eye of a fly appears, under the micro- 

 scope, as a great number of facets 

 hexagonal in shape, each of which 

 represents a single eye. The struc- 

 ture of insects' eyes is a subject too 

 comprehensive to be treated at this 

 time. 



Another method of preparing the 

 eye is as follows: — 



Remove the eye as before de- 

 scribed, and place it in the potash- 

 solution for a few moments and 

 clean it as before. It is not nec- 

 essary to prolong the treatment with 

 potash longer than is necessary 

 to clean it well. After washing, 

 transfer it to the carbolic acid, 

 where it may remain until a cell 

 is prepared for it. The eye is to 

 be mounted in its natural shape, and 

 a cell must be prepared deep enough 

 to receive it. Such a cell may be 

 made in various ways, but, perhaps, 

 the simplest and quickest plan is 

 this : Take a half-inch curtain-ring 

 and flatten it somewhat by hammer- 

 ing it on a flat-iron; cement it to a 

 slide by means of shellac dissolved in 

 alcohol. Such a ring can be pre- 

 pared in a few moments by drying 

 the shellac over a lamp. When the 

 shellac is well hardened, fill the cell 

 with balsam, and place the eye 

 within it, convex side up, place a 

 cover upon it, and press out the 

 superfluous balsam. Then cement 

 the cover down by means of hard 

 balsam dissolved in benzole. When 

 this coat is dry, clean the slide, put a 



coat of shellac over all, and then 

 finish with any black or colored 

 varnish. 



As thus mounted the eye is adapted 

 to the binocular microscope, condens- 

 ing the light upon it from above. 



There are many different kinds of 

 eyes that may be prepared in either 

 of the above ways. The second method 

 is especially adapted to the mounting 

 of opaque parts of insects. Thus, the 

 entire head of a fly may be mounted 

 in this way. Instead of balsam, gly- 

 cerine may be used as the medium 

 for mountingf. 



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Examination of Water. — In the 

 examination of wa er for sanitary pur- 

 poses, the chemist is often at a loss to 

 know whether there are living germs 

 floating about in it, which his analysis 

 cannot delect. This question fre- 

 quently assumes considerable impor- 

 tance, and various methods have been 

 proposed for testing the water — such, 

 for example, as the addition of sugar 

 and exposing it to the light. Dr. 

 Koch, of Berlin, has proposed a 

 method which promises to meet the 

 difficulty. About 2^ per cent, of gel- 

 atin is dissolved in a small quantity of 

 water, and heated to kill whatever 

 germs may be in the solution. It is 

 then mixed with the water to be tested. 

 On standing, wherever there is a spore 

 capable of growing, it is held in sus- 

 pension and forms a centre of growth 

 which can be detected. The gelatin 

 keeps a record, for a time, of the life 

 in the water. 



