1886.] 



MICROSCOPICAL JOURNAL. 



67 



forms ought to be placed close to- 

 gether, and laid, as far as practicable, 

 in the position most suitable to mount- 

 ing afterwards, for instance, Eupodis- 

 bus and similar forms with convex 

 side down, always working with great 

 care, as a single careless motion may 

 destroy the work of hours. By using 

 a piece of cardboard of convenient 

 size, pierced on opposite sides to 

 hold a string, the string to be grasped 

 by the teeth, a very good shield is 

 formed, and all danger from losing 

 or disturbing the selected diatoms by 

 breathing avoided. 



In selecting the robust or convex 

 forms, and preventing loss from flying 

 of the bristle or cover, the cover may 

 be moistened with kerosene diluted 

 with benzine, which may be easily 

 evaporated afterwards. 



To mount the selected forms, the 

 cover has to be coated with a thin 

 film of bleached shellac dissolved in 

 ether, well filtered through bone- 

 l)lack ; place the cover on a warm 

 slide, and apply a small drop of the 

 shellac solution ; attach it and the 

 cover containing the selected material 

 to a slide, and transfer it to the 

 mounting microscope ; moisten the 

 clean cover with kerosene as above, 

 and transfer the diatoms ; place them 

 carefully ; they will move freely on 

 the moist surface. • When the cover 

 is filled, remove it to a slide, evapor- 

 ate the kerosene slowly over a lamp 

 flame. The diatoms will become 

 firmly attached to the softening shel- 

 lac film; apply the medium. Of 

 course alcoholic or chloroform solu- 

 tions are out of question, and benzine, 

 benzole, toluol or xylol solutions have 

 to be used. On mounting the cover, 

 a small drop of the medium must be 

 first placed on the slide, the cover 

 put on it and gently pressed down, 

 and the medium that may exude re- 

 moved with brush moistened with 

 chloroform on the turn-table. Glass or 

 zincfoil cells cemented to the slides 

 afford a safe protection against crush- 

 ing or displacement. 



To attach selectetl forms to the 



cover in dry mounts, pure glycerin 

 diluted with alcohol and distilled 

 water answers well. The film will 

 stay moist a good while, but may be 

 perfectly evaporated by heat after- 

 wards, the diatoms becoming fixed 

 securely. For the more robust forms 

 a little gum-arabic may be added, but 

 care is necessary not to overheat and 

 discolor the gum in solution after- 

 wards. 



Photo-Micrograpliy. — V. 



BY THE EDITOR. 

 [ Continued from page jO-] 

 The focussing of the image upon 

 the ground glass is now to be con- 

 sidered. The importance of careful 

 focussing is obvious to every one, but 

 it shoidd be remembered that in the 

 case of objects of a particular kind, a 

 perfectly sharp focus may not be the 

 correct focus. This is due to the 

 difference between the focus for vision 

 and the focus for photography. Mr. 

 J. D. Cox has shown* that in photo- 

 graphing diatoms he would occasion- 

 ally get a positive photograph instead 

 of a negative, the lights and shadows 

 being re^•ersed, just as they are when 

 we focus up and down while looking 

 at the markings directly. This is 

 particularly noticeable when using 

 lenses not specially made for photog- 

 raphy ; but as it is purely a matter 

 of focus, such errors can be avoided 

 with any lens by moving the objec- 

 tive back from the slide with the fine 

 adjustment, after the image on the 

 screen is sharp to the eye. The ex- 

 tent of the movement should be ascer- 

 tained by experiment, probably not 

 more than half a turn of the milled 

 head will be required, but obviously 

 no rule can be given as it will de- 

 pend upon the lens, the microscope, 

 and the length of the camera. It is 

 luore for low powers than for high. 

 First take a pictui^e of a diatom or 

 other delicate object with the focus 

 sharp to the eye. Then, if the picture 



*The Actinic and Visual Focus in Photo-Microg- 

 raphy, etc., vol. vi, p. 193. 



