APPAKATUS FOR HOLDING SPECIMENS 



53 



of placing the object between a glass slip and a tbin cover 

 glass. Such glass slips are made 3 inches long and 1 inch 

 wide, and it is only for a few special pur- 

 poses that slips of any other size are used. 

 The thickness of such slips varies from J to 

 1 J mm. Most forms of illuminating apparatus 

 can be adjusted to focus through slips of 

 such thickness, but apparatus which cannot be 

 focussed is constructed for slips of a thickness 

 of 1 mm., which must be specially selected. 



Cover glass is a specially thin form of glass 

 prepared for use with the microscope. It is 

 made in squares or circles of 5/8 to 7/8 inch 

 diameter, or can be cut to any particular size 

 required. It is made in three thicknesses : 



No. 1. Average thickness . "006 in. '15 mm. 

 2. „ „ . -008 „ -2 „ 



o. ,, ,, . *Ui ,, 'JiO ,, 



Cover glass. 





Fig. 47. — Thin 

 Glass. 



The thickness varies about 20 per cent, in different individual Measuring 

 pieces, and absolute uniformity of thickness can only be obtained "''^^^ ^^^^* 

 by selection. A screw micrometer is the most useful form of 

 appliance for measuring cover glasses. 



Cover glass can also be measured by the microscope itself. 

 The fine adjustment milled head of a microscope is provided 

 with a series of divisions, and the amount that the body tube of 

 the microscope is moved by the revolution of the milled head for 

 one division is given on page 96. A high-power dry object 

 glass should be used, and the cover glass to be measured placed 

 under the microscope, resting on a glass slip so that one edge 

 of the cover glass appears near the centre of the field of 

 view. The microscope should now be carefully focussed on 

 to specks of dust on the upper surface of the cover glass 

 and the position of the fine adjustment milled head ob- 

 served. The milled head provided with the divisions should 

 then be turned till the dust on the slip is in focus and the 

 number of divisions that the milled head has moved to make 

 the alteration noted. This number multiplied by the value 

 of one division gives the thickness of the cover glass. It is 

 necessary to focus particles of dust which are situated on 

 the slip to one side of the cover glass, and not seen through 

 it, as the optical path seen through glass is not the same as that 

 in air. 



If it is desired to ascertain the thickness of a cover glass of Measuring 

 a mounted specimen where the edge of the cover glass cannot ^^^e^jP^sg^ 

 be observed, the microscope may be focussed to the dust on the ot mounted 

 surface of the cover glass and then to the object itself, but the^^^^^'" 

 result so obtained will be too small, and must have one-half as 



