INTRODUCTION. XXIX 



an inch. Zeiss has one at 10s. ruled up to y^ of a millimetre, 

 i.e., about ^^^ of an inch. 



Any steady table can be used by the microscopist for 

 working, but it should be looked to that it is not too small, 

 and not polished or varnished on its surface. This surface is 

 best painted a dull dark colour. The table is so placed that 

 if possible the microscope faces a window, and at not less than 

 one yard distance from it. Any position of the window is good 

 which allows a free outlook. From direct sunlight we pro- 

 tect ourselves by a white roller blind, which is best made of 

 tracing-linen. The dazzling white light which we obtain 

 when the direct sunlight plays upon the blind gives the most 

 favourable conditions for observation with high powers. In 

 using the microscope with light taken from a window, light 

 coming from any other direction should be avoided. If a 

 white roller blind is used to pass direct sunlight through, the 

 eyes should be protected in some way from its glare. If 

 artificial light is needed, microscope makers provide various 

 forms of oil-lamp specially constructed for use. The light of 

 these is very often improved by passing through a Florence 

 flask filled with dilute ammoniacal cupric oxide, or through 

 a disk of bright blue glass placed above the mirror. Artificial 

 light, partly on account of the brightness of the field of view 

 relatively to that of the room generally, and partly owing to 

 the strength of its shadows, is very tiring to the eyes, and 

 every care should be taken, in using it, to have the room 

 generally as brightly illuminated as is the field of view. 



The necessary object slides and cover-glasses can be 

 obtained of most opticians. The former are procurable with 

 either unground or ground edges at a cost varying from '2s. 

 to 4s. per gross respectively. They are three inches long by 

 one broad. The cover-glasses for ordinary observation 

 should be about ^-inch square ; but the observer should also 

 have larger ones for specially large objects or series, and also 

 others somewhat smaller (f-inch square) which will usually 

 suffice for permanent preparations. If we use powerful 

 objectives, it will be best to obtain these cover-glasses of 

 definite thickness. For the beginner, this is not of special 

 importance ; but the more advanced student will find it 



