SUNDRY APPARATUS 69 



stage micrometer. For this purpose, focus the scale of a stage 

 micrometer carefully ; if 1/lOth of a millimetre now measures 

 2-5 mm. in the scale with the correct tube length of 160 mm. 

 and a particular object glass, the magnifying power of that object 

 glass is 25. 



The first image formed by a microscope is produced by the initial 

 object glass at a position rather above the stop of the eyepiece. Sfwer^^'"^ 

 This initial magnification depends on the focal length of the object 

 glass, and also the position of this image, which is governed by 

 the length of tube of the microscope. The approximate initial 

 magnifying power of each object glass or the enlargement pro- 

 duced in the first image is engraved on each Beck object glass 

 for a standard tube length of 160 mm. It can only be approxi- 

 mate, because different eyepieces have their stops in slightly 

 different positions, and therefore a small variation in the theoreti- 

 cal tube lengths is caused by the use of different eyepieces. 

 The eyepiece magnifies the first image formed by the object glass 

 by a fixed amount, according to the focal length of the eyepiece, 

 and does not vary, and the total magnifying power at the 160- 

 mm. tube length is obtained by multiplying the power of the 

 object glass by the power of the eyepiece. 



The Beck micrometer eyepiece measures the size of the first 

 image formed by the object glass in millimetres and tenths of 

 a millimetre. The result obtained when the drawtube has been 

 set at 160 mm. has only to be divided by the initial magnifying 

 power of the object glass to give the actual size of the object 

 being measured. 



Small variations may occur in individual lenses, but they 

 are usually not sufficiently great to be of consequence in ordinary 

 work. 



The camera lucida is an apparatus for making correct drawings. Horizontal 

 It is made in four models, suitable for three different positions Sda^ 

 of the microscope. ' 



The Beck horizontal camera lucida (Fig. 77) requires the 

 tube of the microscope to be in a horizontal position, and the 

 paper upon which , 



the drawing is to j^^^^^ ^^^^r ' 



eyepiece of the ^l^^MLg^y "J" 



microscope. The ^^^^^^^^^ « 



camera lucida is a • 



small half - silvered Fig. 77. — ^No. 3368, Horizontal Camera Lucida. 



prism held in a 



mount which fits on to the drawtube of the microscope in 

 such a position that one surface is close to the front lens of 

 the eyepiece. The observer places his eye immediately above 

 the prism, and the image seen in the microscope is reflected 



