866 SUMMARY OF CURRENT RESEARCHES RELATING TO 



be found in the use of different coloured lights produced by tinted 

 glasses, carefully adapted to the intensity and colour of the staining. 

 The use of glass, or of solutions of a colour complementary to that 

 of the object, has been long employed in the arts in reproducing 

 paintings. Koch, in his ' Traumatic and Infective Diseases,' relates 

 his experiences with this method, but condemns it as impracticable. 

 On account of the length of exposure and vibration " the picture does 

 not have sharpness of outline sufficient to enable it to be of use as a 

 substitute for a drawing, or, indeed, even as evidence of what one 

 sees." * 



Notwithstanding the unfavourable experience of this skilled in- 

 vestigator, some subsequent results by this method have been most 

 encouraging. Defrenne obtained excellent pbotographs of the Bacillus 

 tuberculosis by means of fuchsin staining and green glass, and quite 

 recently the author's own experience with this same bacterium and 

 stain have been very gratifying. Since then a number of modifica- 

 tions have been tried. As a result of these experiments the practical 

 deductions have been reached, that when the staining and thickness 

 of the specimen are insufficient to give the necessary actinic contrast 

 with the colour of the field, we can best succeed by employing a 

 coloured glass, whose tint will be such as to give the contrast, as well 

 as to afford light to sufficiently impress the plate where not occupied 

 by the object. Such a colour will not be the complementary one in 

 many instances. With blue stainings the use of the complementary 

 yellow would yield but a faint image, since the weak actinic power of 

 the transmitted rays are insufficient to deeply affect the unoccupied 

 parts of the field. The substitution, however, of a suitable shade of 

 green affords sufficient contrast of the object as well as permits tho 

 passage of rays sufficiently actinically powerful to adequately impress 

 the surrounding parts of the plate. 



With all these colours the exposure is greatly lengthened, with a 

 medium green it being five to seven times longer than with blue 

 light ; as, however, the normal exposure is seldom over one second, 

 the increase has practically little disadvantage. Not only for very 

 minute objects, as bacteria, stained with methyl-blue, under high 

 power, but equally for very thin hematoxylin or carmine sections 

 under low amplification, has this green glass proved most useful. By 

 its use it is always possible to obtain pictures, where all the merits of 

 vigorous negatives with the beautifully sharp details alone obtainable 

 from the thinnest sections are combined, and where the usual method 

 yields but a weak image. 



These suggestions apply especially to sunlight. To those engaged 

 in such work, who have never employed these means, the shades of 

 green offer themselves as valuable modifications of illumination well 

 worthy of a trial. The exact time required — a matter of importance 

 — must be determined for existing conditions by each manipulator. 



Mr. J. W. Queen suggests f a trial of the stained gelatin plates 

 now coming into use for the purpose of securing contrast. The 



* Magnin and Sternberg's Bacteria, 2nd ed., 1884, p. 195. 

 t Micr. Bulletin (Queen's), iii. (1886) p. 32. 



