700 SUMMARY OF CUREENT RESEARCHES RELATING TO 



" In the discussion which followed the termination of Dr. Sudduth's 

 talk, Dr. Allen admitted that much credit was due to Dr. Sudduth for 

 his success in colouring slides, but whilst the staining of specimens was 

 of value as aiding the diiferentiation of tissues, he, Dr. Allen, could not 

 see what was gained by colouring slides. Whilst this is undoubtedly 

 true, the plain photomicrograph being perfectly intelligible to the trained 

 eye, it was the general opinion among the members present that the 

 coloured pictures were more satisfactory to those not so well acquainted 

 with the tissues." 



Photomicrography and the Chromo-copper Light-filter.* — Dr. E. 

 Zeltnow claims that his light-filter fulfils the two conditions required of 

 it, namely, it only allows rays clearly visible to the eye and those of a 

 definite wave-length to pass through, and when used in a concentrated 

 form wave-lengths of from 570 to 550 only traverse the filter, so that 

 the light may be fairly called monochromatic. With ordinary objectives 

 perfectly sharp negatives are obtained. The filter is made by dissolving 

 160 grm. of pure dry nitrate of copper and 14 grm. of pure chromic acid 

 in water up to 250 ccm. A solution more easily made and sufficient for 

 almost all cases in a layer of 1 to 2 cm. thick is composed of 175 grm. 

 sulphate of copper, 17 grm. bichromate of potash, and 2 ccm. sulphuric 

 acid in water up to 1/2 litre. With a mineral-oil lamp the latter fluid 

 may be diluted with an equal or double volume of water. 



Since ordinary dry plates are but little sensitive to light which has 

 passed through this filter, erythrosin plates must be used. These are 

 produced by bathing the former in a weak solution of erythrosin {1 grm. 

 erythrosin dissolved in 500 ccm. spirit and 5 ccm. of this solution with 

 200 ccm. of water are used for each bath). These plates will only keep 

 for three or four weeks at the most, but erythrosin plates can be obtained 

 from the makers which will last from three to six months. Owing to 

 ihe erythrosin the plates are very sensitive to yellow-green rays with a 

 wave-length of 560. 



A fluid very similar in outward appearance to the chromo-copper 

 filter can be made by the supersaturation of copper salts with ammonia 

 and dilution with chromate of potash. This, however, only allows such 

 green rays (510-455) to which the erythrosin plates are little sensitive 

 to pass through. Preparations stained red, blue, green, blue and violet, 

 are easily photographed by aid of the chromo-copper filter, since in 

 consequence of the extinction of these colours the preparations appear 

 black on a green ground. 



Simmons, W. J. — Magnification in Photomicrography. 



Amer. Mon. Micr. Journ., X. (1889) p. 180. 



C5) Microscopical Optics and MauipxQatiou. 



Simple Apparatus for measuring the Magnification of Optical 

 Instruments-! — The usual method of measuring magnifying power con- 

 sists in comparing the image of an object of known dimensions, seen by 

 one eye through the instrument, with another object seen at the same 

 time by the other eye. By a simple optical arrangement, however, both 

 can be seen simultaneously by the same eye, which is the principle of 

 the apparatus constructed by Dr. A. Oberbeck. Two rectangular mirrors 



* Eder's Jahrb. f. Photogr. u. Eeproductionstechnik, 1889. 

 t Central. -Ztg. f. Opt. u. Mech,, x. (1889) pp. 176-7 (3 figs.). 



