19 



without the use of considerable force. It is to the under sur- 

 face of this transverse piece thus held in position by the sand- 

 paper surfaces at the ends of the screen, that the drawing paper 

 is attached by means of ordinary thumb tacks. This provision 

 is advisable, as in the use of the projector it is often necessary 

 to bring the drawing into register, or to change the register 

 somewhat. By slightly lifting the sand-paper surfaces, the paper 

 is easily changed in position to accommodate any change of 

 register. It is usual to make two punctures on the negative as 

 register marks. As it is anticipated that this apparatus will come 

 into much wider use, it may not be out of place to mention a 

 number of points which experience has shown to be worthy of 

 imitation or improvement. The light is usually a very strong 

 one and in consequence the operator is most strongly illumin- 

 ated. It is therefore desirable that he should wear dark- 

 colored clothing, otherwise the reflections from his person will 

 cau-se considerable interference. It is also desirable to have the 

 room as dark as it can be conveniently made — the darker the 

 better, especially when high power magnifications are in pro- 

 gress. When the finest detail is to be drawn, it is also advisable 

 to arrange at the top of the screen a roller blind carrying an 

 absolutely opaque blind with an aperture not mucli larger than 

 will give freedom of movement to the hand in drawing, say 

 from four to eig'ht inches across. This shuts out all of the extra 

 illumination and the eye will then accommodate itself in such a 

 manner that any fine detail is much more readily seen. It is 

 also desirable to have a drop blind by which the whole of the 

 lig'ht may be instantly shut ofif, as well as a liglit at the back 

 which can be also instantly let on so as to give the operator a 

 very distinct notion of what he is producing. For it must be 

 pointed out that when the drawing is being made, it is not 

 easy for the operator to see his work, especially if he is working 

 with a blue pencil, as is often the case if the drawing is to be 

 afterwards finis'hed up, photographed, and reduced for illus- 

 tration purposes. 



The illustration room is also furnished with an ordinary 

 camera working on vertical uprights of the same character as 

 those already described in connection with the microscope win- 

 dow. These are a second pair of steel girders imbedded in 

 cement below the building and coming through the floor with- 

 out contact. It is found that a great variety of objects are more 

 conveniently photographed by using the camera in this manner 

 than in any other, though of course it is necessary for many 

 classes of objects to use it in the usual way. To facilitate the 

 production of the necessary backgrounds, a well is provided. 



