504 SUMMAKY OF CURRENT RESEARCHES RELATING TO 



anyone who makes a trial. The method found most effective in this 

 laboratory is that of using a thin black tissue-paper, which is blued on 

 the under side. A piece of enamelled board of suitable size for the 

 drawing is placed on the drawing board — i.e. the right-hand leg-of- 

 mutton table — and it is then covered with the black tissue-paper, with 

 the blue side down. A tracing is now made with a white ivory point. 

 This results in the production of a blue outline drawing on the enamelled 

 board. This sketch is put aside for further reference, or for the produc- 

 tion of a finished drawing whenever necessary, or may be finished up at 

 once. The object to be secured in this blue sketch is a sufficiently 

 good representation of the object to be illustrated, which shall have 

 sufficient size to admit of a liberal reduction when the drawing is photo- 

 graphed on metal preparatory to etching. Thus, if it is desired to 

 publish an illustration having a magnification of 500 diameters, it is 

 advisable to produce a blue sketch at from 1000 to 2000 diameters. 

 This is easily accomplished with the apparatus that has been described. 

 By placing the prism reflector at a considerable horizontal distance from 

 the eye-piece of the Microscope, say 1 foot, and lowering the right 

 hand leg-of-mutton shaped table sufficiently, magnifications of liberal 

 dimensions are easily secured. Needless to say, the production of a large 

 coarse drawing is an easier matter than the production of the same 

 drawing on a smaller scale ; so that the operation is not only better, but 

 considerably easier if carried out in the manner described. It is un- 

 necessary to go into the details of converting the blue sketch into a 

 pen-and-ink drawing. These present no peculiarities. It ought, per- 

 haps, to be mentioned that the object of using the blue colour is to 

 avoid trouble through the alterations that may be necessary in finishing 

 the drawing. Any fight blue lines which are left on the enamelled 

 board need not be removed, as they do not affect the sensitive photo- 

 graphic film sufficiently to cause any inconvenience in the production of 

 an etched block. The black tissue-paper mentioned is produced by 

 inking ordinary tissue. The ordinary blue carbon paper gives too dark 

 a blue to meet the requirements. The blackened tissue is rubbed on 

 one side with dry Prussian blue powder. This gives a light blue tracing. 

 At an earlier stage it has been mentioned that all the accessories in 

 connection with the Microscope are painted black. In addition to this 

 precaution, such arrangements are made that the room itself can be 

 darkened, in fact, converted into a photographic dark-room at will. 

 This object is secured by having all the window blind connections light- 

 tight. The oblong aperture, about 5 in. by 8 in., through which the 

 Microscope receives its light, is screened by means of several thicknesses 

 of flexible black cloth made into the form of a sleeve. This cloth sleeve 

 attached around the perimeter of the opening, is notched above, so that 

 it surrounds the Microscope just beneath the stage, and buttons on to 

 one of the screws at the back of the Microscope. No light reaches the 

 eye except that which comes through the instrument. If, now, the slide 

 in front of the large glazed aperture be closed and the room be darkened, 

 the operator sits in absolute darkness. Any one who has had experience 

 with a photographic dark-room, must have observed how, after a period 

 of from five to ten minutes therein, the eye becomes accustomed to the 



