ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 229 



Photomicrographs in Colour.* — H. R. Eggleston describes the 

 process of making lantern-slides representing photomicrographs of 

 stained sections. The process is as follows : — Lantern-plates are sen- 

 sitized by bathing for five minutes in a 2^ p.c. solution of ammonium 

 bichromate containing 5 c.cm. of ammonia to the litre, the temperature 

 of the bath being not above 60° F. The plates are then rinsed for two 

 or three seconds in clean water, drained and dried as uniformly as 

 possible, being kept in the dark while drying. The sensitized, plates are 

 then exposed through the glass under the negative to the light of an 

 arc lamp, the average exposure being about three minutes at IS inches 

 distance. The exposed plates are then developed by rocking in trays of 

 water at about 120° F. until all soluble gelatin is removed. The plates 

 are then rinsed in cold water, fixed in hypo, and washed free of the hypo. 

 They are then ready for staining. The staining is done with a 1 p.c. 

 solution of dye containing 1 p.c. of acetic acid, the dye being selected to 

 simulate most closely the original stain of the section. When sections 

 stained with two different colours are being photographed, negatives are 

 made through suitable colour-filters, and are then dyed in the two stains 

 and placed face to face, so that a two-colour slide is obtained. Suppose 

 a section is stained red and green Two negatives are made on panchro- 

 matic plates — one with a red filter, which will cause the green to appear 

 as clear spaces in the negative and will not record the red, and the other 

 with a green filter, which will record the red and not the green. The slides 

 made from bichromated gelatin are stained — -that from the red negative 

 with the original green stain, and that from the green negative with the 

 original red stain. The filters used are Wratten M filters. The choice 

 of the filter is decided by visual trial. Thus photographing a section 

 stained with htematoxylin and eosin the A (red) filter shows no trace of 

 the eosin and gives a good strong negative of the hfematoxylin. The 

 B and C filters are used together for the other negative, giving a blue- 

 green colour and recording the eosin and haematoxylin both fully, and 

 from these two negatives positives are made and stained with a blue and 

 a red dye. 



C5) Microscopical Optics and Manipulation. 



Dividing-engine for Ruling Diffraction Gratings.f — • " Xature " 

 records that part 1 of vol. xxx. of the Proceedings of the Royal Society 

 of Victoria contains a description of a new dividing-engine for ruling 

 diffraction gratings by J, H. Grayson, of the University of Melbourne. 

 The design and construction of this machine have occupied Grayson, 

 whose skill in work of this type is well known, for seven years, and 

 the completion of the task places spectroscopists under a great debt of 

 gratitude to him. His paper contains a detailed description of the 

 machine, and gives full particulars of the methods used for grinding 

 and testing the screw\ The machine is set up in a room of its own 

 in the basement of the University, and is driven by a ^^ h.-p. hot-air 

 engine placed in an adjoining room. Ruling-diamonds are broken 

 stones, in which the fracture along a cleavage-plane intersects an outer 



* Trans. Amer. Micr. Soc, xxxvi. (1917) pp. 279-81. 

 t Nature, March 21,' 1918, p. 51. 



