ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 489 



liglit, which, enables the microscopist, without very much trouble, to secure 

 at any moment a photomicrograph of the object he is examining, and with 

 only a few minutes' delay ; while formerly it was almost necessary to utilize 

 sunlight either with or without an equatorially mounted prism, or some 

 form of heliostat, or the solar Microscope; for the magnesium, oxyhydrogen, 

 and electric lights, though so useful, never obtained more than a temporary 

 claim. Looking to the quality of the results, possibly the palm would be 

 granted to the wet collodion process, as gelatino-bromide negatives often 

 show a fine granulation, absent in the collodion or albumen film, which 

 interferes with enlargement. Still the advantages for general work lie 

 with the dry bromide plate, which is the process the author adopts. All 

 who have endeavoured to obtain the bast results with the gelatino-bromide 

 plates have from their great sensitiveness found a difficulty both in the time 

 of exposure and the mode of illumination, and it is to both of these that 

 the author devotes considerable attention, and introduces a more certain 

 way to regulate the exposure according to the non-actinicity of 

 the object, whether due to thickness or colour, to which may be added 

 the difficulty occasioned by alteration in distance between the object 

 and the scr'?en, from a different manufacture of the plates, and from 

 the use of different objectives of the same power. To meet these dif- 

 ficulties the author has constructed a scale or table by which to regulate 

 the time of exposure under these different circumstances. This table of 

 exposures has been ingeniously founded upon the visibility of the figures 

 on Warnerke's sensitometer under the same illumination, and at the same 

 distance of the screen as the gelatino-bromide plate will be placed at, as 

 one of the terms, and used in conjunction with the known scale of the 

 sensitiveness of the plates, either as stated by the maker, or as tested on 

 trial with the same sensitometer, as the other term. These two terms or 

 readings being known, the third, the time of exposure required in seconds 

 for such a plate to be properly exposed, is indicated in the table up to ten 

 minutes. Examples of the use of the scale are given, and every photo- 

 micrographer who wishes to work upon this, the most sure method of 

 exposure yet devised, will heartily thank the author for his effort to 

 supply a deficiency, which even long years of experience could not always 

 obviate without the loss of a plate or two. 



Dr. Bousfield rightly lays great stress upon the method of illumination 

 when using a paraffin lamp, and points out the correct way of obtaining a 

 brilliant field, or for securing a dark-ground illumination. It may here 

 be noticed, in connection with this latter method of illumination, that stereo- 

 scopic photomicrography is passed over in silence, which we should have 

 been glad to see noticed. Part of a chapter is devoted to the use of " ortho- 

 chromatic" or " isochromatic " plates, with the use of tinted glass between 

 the bull's-eye and condenser, to produce in the negative actinic contrast 

 between the different parts of the object, inter se, and the background, and 

 the author furnishes the^ following rule, " to use such a coloured screen as 

 reduces the colour of the object to a neutral tint," a table being given of 

 the different colours found most useful, and the number of seconds the time 

 of exposure must be increased. 



Many years since. Dr. Maddox, instead of using coloured glasseSj 

 employed coloured varnishes applied to the back of the slide, thus getting 

 rid of one reflecting siu'face, and later he tried the use of a small globe 

 filled with various coloured media and placed between the bull's-eye and 

 substage condenser, but nearer the latter, thus obtaining a further concen- 

 tration of the light. 



The author seems to lean to the use of the eye-piece combined with the 



1887. ^ K 



