PHOTO-MICROdllAPHY AND PHOTO- 

 MK'UOMETKY. 



(Plates Y.— VIII.) 



By JOHN THOMSON, M.B. (President). 



(Bead before the Eotjal Societii of Qxu'cnsland, 20th October, 1900.) 



The ordinary working microscopist seldom refers to the ampli- 

 fication he is obtaining, but speaks of the objective and ocular 

 he employs. He does not bother much with a stage or eye- 

 piece micrometer — nor need he in his every day task — but 

 should he desire to record his observations, either by drawing 

 or by photography, it is important he should know the 

 magnifications given by certain combinations and arrangements 

 of his apparatus, and better still if these magnifications be of 

 recognised standards. 



Looking at many, perhaps most, of the reproductions found 

 in microscopical literature, one cannot but recognise their use- 

 lessness for comparison purposes. Some, chiefly the drawings, 

 represent scales difficult to calculate, such as 140 or 330 

 diameters, perhaps with the units added as 143 or 337 ; others, 

 mainly photographs, although originally perhaps of the value 

 attached to them, as 500, 750, or 1000 diameters, have suffered 

 at the hands of the photo-mechanical printer, and losing their 

 true dimensions have ceased to be standards. 



As so much educational work is now done with the optical 

 lantern, its 3J x3|- slide seems to be accepted as the recognised 

 size of photo-micrographs, whether on glass or paper ; and the 

 usual masks having circular openings of 2-|, 2^-^, and 2 inches 

 respectively appear admirably suited for the micrographs 

 referred to. 



With the lower powers it may be impossible to insist on 

 the use of standard amplifications, for something has to be 



