USE OF THE COMPOUND MICROSCOPE 31 



If, after raising the objective to its proper limits, 

 the object does not appear, it will be because it is 

 not properly centred in the field. The " finding " 

 of an object with lower powers is simple enough ; it 

 becomes more difficult with high powers, whose 

 -gpertUTB is very small. The beginner should 

 practise focussing first with a low power, and when 

 expert with it proceed to the use of high power. 

 It is also advisable to practise with a permanently 

 mounted object, which any dealer will supply. 



The subject of the source of illumination needs 

 a little consideration. Some workers use daylight, 

 but the beginner is warned against using direct 

 sunlight, a proceeding which might lead to damage 

 to the objective as well as to a mounted object. 

 If daylight be used, it should preferably come 

 through a window facing north. But there are 

 two distinct disadvantages in the use of daylight : 

 in the first place, it is a variable quantity ; and, in 

 the second, it illuminates the workroom as well as 

 the object. Work is always done at its best when 

 the worker is practically in darkness and only the 

 object is illuminated. However, with low powers 

 daylight does not present any difficulties. 



But it is generally more convenient to work by 

 artificial light, and it is really the more satisfactory. 

 I sometimes use electric light, but find that the 

 greatest satisfaction is got from a small paraffin 

 lamp taking a J-inch flat wick. Elaborate micro- 

 scope lamps with many conveniences are advertised 

 by dealers, but a glass lamp, which need cost no 

 more than Is., will meet all the requirements of a 

 beginner. The flame is made whiter by placing a 



