THE LACTOSCOPE. 



157 



produce beautiful colours. The whole subject is very interesting to the 

 scientist, but rather a difficult one for the general reader to understand. 



Amongst the uses to which light has been put is that of a milk-tester. 

 The LACTOSCOPE will show the quantity of butter contained in a certain 

 quantity of milk, by diluting it till it displays a certain degree of transparency. 

 There is another method, by the transmission of light. 



The first test is obtained by means of a glass tube about nine inches 

 long, closed at one end, and containing a small porcelain rod marked with 

 black lines. A small quantity of milk is measured and placed in the tube. 

 The blac!: lines cannot at first be seen through the tube, but by adding 

 water the milk is rendered transparent, and the black lines become visible. 

 The surface of milk in the tube, by a graduated scale upon it, shows the 

 percentage of butter. 



The second method is not so simple. A short tube of tin, blackened 

 on the inside, and supported upright, has an opening on one side, and 



Fig. 167. Cut card figures. 



opposite this, inside the tube, is a mirror placed at an angle of 45. "By 

 placing a lighted candle at a known distance opposite the opening, its light 

 is reflected in the mirror and thrown upward through the tube. On top of 

 the tube is placed a round vessel of glass or metal, closed at the bottom by 

 a sheet of clear glass. The vessel is closed at the top by a cover having 

 an opening in the centre, in which slides up and down a small tube closed 

 at the bottom with glass, and having an eye-piece at the top. The milk to 

 be tested is placed in this vessel on the top of the tin tube, so that the light 

 of the candle reflected from the mirror passes upward through the milk. 

 Then, by looking through the sliding tube and moving it up and down, a 

 point may be found where the image of the candle in the mirror can 

 be seen through the milk. This device depends, as will be seen, on 

 observing the light transmitted through a film of milk, and the thickness of 

 the film is the measure of the value of the milk. The movable tube 

 contains a graduated scale, and by comparison of this with a printed table, 

 the percentage of butter in the milk may be ascertained." 



In concluding this chapter we give a few hints for some pleasant 



