92 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



screw upon a lever. The imlled head on one side moves the body at 

 twice the speed of that on the other, so that either a nioderatelj fine or 

 a superlatively fine adjustment can be used. The friction is so slight 

 that the delicacy obtained only by a lever and point contact motion is 

 secured. One of the milled heads is divided in -yho of a millimetre for 

 measuring thickness. 



The mirror is 2 in. in diameter, and is plane on one side and concave 

 on the other, mounted on a swinging arm on which it slides up and 

 down for focusing.' 



The substage is made to the standard 1*52 in. diameter. 



A possessor of a Standard London Microscope in its simplest form 

 with only a tubular substage can, at any time, obtain one of the different 

 substages together with full instructions for fitting, and can attach it 

 himself with the aid of a screw-driver. 



Beck Micrometer Eye-piece. — This consists of a complete eye-piece 

 with a magnifying povrer X8, and a special vernier millimetre scale A 

 (see diagram) placed in its focus, which is outside the lenses. 



It is provided with a collar B, which fits over the draw tube and can 



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-'B 



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be clamped in position by a milled head C. The eye-piece itself can be 

 focused up and down by revolving it in its fitting till the scale A is in 

 exact focus for the observer's eye. 



The scale (see diagram) is in millimetres with a vernier reading 

 to yV of a millimetre. 



On the left is a vertical series of divisions divided in half -millimetres 

 for rough measurement. For fine measurement the object to be 

 measured is placed in a horizontal position, or the eye-piece scale turned 

 round, and the length is measured in y^u millimetre by use of the slanting 

 line on the right. The image of an object as shown in the diagram 

 measures 8*25 millimetres, because it covers three large divisions and 

 extends to the oblique line at a point halfway between the • 2 and • 3 

 of tenth millimetre vernier divisions. 



To obtain the actual size of the object itself this result has merely 

 to be divided by the initial magnifying power of the object glass, which 

 is engraved on each object-glass. 



In cases where great accuracy is required each object glass can be 

 verified as to its initial magnifying power by the use of a stage micro- 

 meter. For this purpose focus the scale of a stage micrometer carefully ; 



