GENEEAL PKELIMINAEY PEEPAEATIONS 185 



where a screen is handled a good deal, this becomes a serious 

 disadvantage. 



The finest focussing-screens are those made by chemical 

 means. Take an ordinary photographic plate, preferably an 

 extra-rapid one, and without any preliminary exposure to light, 

 immerse it in the dark in any non-staining developer free 

 from potassic or ammonic bromide. The plate may be left in 

 this for from five to twenty minutes. The longer periods result 

 in a fine and translucent screen, and the shorter periods in a 

 screen that is equally fine and free from granularity, but less 

 translucent. At the end of this period of development, the 

 plate has to be washed and fixed in the ordinary way. It 

 should then be immersed in a solution of iodine in potassic 

 iodide ; the exact strength of this solution is not important. It 

 may be left in for some few minutes until the plate is com- 

 pletely iodised, then again washed and immersed in dilute 

 ammonia, and finally washed and put to dry. This screen has, 

 in the hands of the writer, proved extremely useful when work 

 of a critical nature is being carried out and when the plain-glass 

 screen does not fulfil its purpose. If used in conjunction with a 

 focussing-ocular, the grain is so fine that there is no evidence 

 of it, even if the ocular is a fairly high-power one. The difficulty 

 of accommodation that occurs in some cases when using a 

 plain-glass screen, is entirely absent when some such fine-grain 

 screens, giving a concrete image, are used. 



The ground-glass screen may be rubbed over with a small 

 quantity of oil in cases where it is too coarse, or where a feeble 

 source of light is used and it is not sufficiently translucent. It 

 need hardly be mentioned that the ground-surface of the ground- 

 glass screen, or in the case of the plain screen, the surface which 

 has the diamond scratches on it, should be towards the light. 



An extremely useful accessory is a white opaque screen, 

 placed in a suitable position so that the microscopic image can 

 be projected on to it, and this image on the screen observed 

 to see if the centration or other adjustments of the illuminant 

 or optical system are correct. With an apparatus of the type 

 in which the microscope and accessories swing out of the 

 optical centre, the image can easily be projected on to such 

 a white screen if placed in a position at either side of the 

 camera back. 



