912 SUMMARY OF CURRENT RESEARCHES RELATING TO 



by staining destroys the natural appearance, and the picture no longer 

 conveys the idea that the object is transparent. It is consequently 

 brought nearer to the levql of a woodcut and to a certain extent loses 

 its value as a photo-micrograph. 



Unicellular Algce. — These should be mounted for photography in 

 very shallow cells, made by turning a circle of white-zinc cement 

 upon a slide. Their colour and natural appearance will be preserved 

 in an aqueous medium, such as weak carbolic-acid water or camphor 

 water. Unfortunately, photography cannot reproduce the rich ruby 

 colour of ProtoGoccus nivalis, or the bright green of Protococcus viridis. 

 The deeply coloured protoplasm of the former arrests light entirely, 

 and we have in a positive print only a uniformly black mass with 

 circular outline, surrounded by another line representing the cell-wall, 

 to delineate the beautiful little ruby sphere with its more or less 

 granular contents. The green colour of P. viridis is better adapted 

 for photography. 



Infusoria. — Many of the Infusoria may be successfully photo- 

 graphed, but it will be necessary to exercise great care in the 

 preparation of slides for this purpose. Generally but a single 

 individual should be in the field of view, and this should be a perfect 

 specimen ; for it is difficult to obtain fields containing several indi- 

 viduals all in the same plane, and in order to show cilia, flagella, and 

 interior details of structure, high powers and very careful focusing 

 will be required. 



Occasionally a living Infusorian may be quiet long enough to 

 have its photograph taken ; but usually the Infusoria are in rapid 

 motion, and it will be necessary to arrest this motion by means of 

 some chemical agent fatal to their vitality. A weak solution of 

 iodine does this very effectually, and at the same time stains the 

 protoplasm a brownish colour. A ciliated Infusorian killed by adding 

 a drop of this solution (iodine 1 gr., potassic iodide 2 grs., water 

 100 grs.) to a drop of the fluid in which it is swimming, remains for 

 a time as if suddenly frozen, with its cilia rigid, and projecting like 

 rays, from the surface of the body. This is a favourable time for 

 photographing the creature, as, later, it is liable to undergo changes 

 which destroy the internal structure. 



Another method is to place a drop of fluid containing the Infu- 

 soria in the centre of a clean glass slide, and to invert this over the 

 mouth of a bottle containing a 1 per cent solution of osmic acid. A 

 very brief time is sufficient to destroy the life of the Infusoria, which 

 may then be selected under a low power and transferred to a drop of 

 clean water. They must be mounted in the thinnest possible stratum 

 of fluid, otherwise they are likely to change position while the 

 exposure is being made. 



As a general rule, transparent objects, like AmcebcB and the Infu- 

 soria generally, should be mounted in an aqueous medium for photo- 

 graphy, as this gives better photographic contrast than does a medium 

 having a higher index of refraction, such as glycerin. 



Spores of Fungi. — The spores of many of the fungi are suitable 

 microscopic objects to photograph, and a photographic method could 



