BY JOHN THOMSON, M.B. 63 



to 140 of its scale — projection eye-piece No. 4 set to 2 on its 

 collar — and a bellows extension giving 29 -^r inches between the 

 stage and the focussing screen. 



All photo-micrographs obtained under these conditions will 

 represent a magnification of 1000 diameters ; but it may be 

 necessary to prove this, to demonstrate it to others, or to find a 

 means of measuring and comparing other prints of similar 

 amplification, and by means of another scale this can be effected. 



The basis of this scale is the micron, known by the symbol 

 jx — the unit of microscopic measurement — the 1,000,000th of a 

 metre, or the 1000th of a millimetre, or, practically, the 

 TTsTTro^ti of an inch ; and yet there are many objects constantly 

 under observation which are but fractional parts of a micwn. 



To prepare this scale, draw on a smooth surfaced piece of 

 Bristol board an area of 10 inches square, and divide it vertically 

 and horizontally by 100 lines, each 10th line both ways, being 

 intense. Reduce this scale by the ordinary methods of photo- 

 graphy until each 100th division is exactly millimetric, and each 

 10th, or broad line, centimetric. The millimetres on the 

 negatives are equivalent to tnio'd magnified 1000 diametres. A 

 glass positive or transparency from this, when varnished or 

 collodionised for protection, can be used as a scale, and if the 

 varnished surface is imposed on the object to be measured there 

 is an absence of parallax. This scale can also be printed on 

 the same glass or paper positive as, and with, the object, and 

 the dimensions of the latter are at once, and graphically, 

 apparent. This double process is very simple. Submit the 

 glass or paper to the scale negative, by contact, in a printing 

 frame, with a short exposure ; then change the glass or paper to 

 the object negative, and give a full exposure ; develop, and the 

 results should be satisfactory. Of course negatives are required 

 of sufficient density to give contrast. 



I need hardly add that the scale I have described is for a 

 magnification of 1000 diameters, and that any other amplification 

 must have its special scale. 



