Processes in the Useful Arts. 157 



pressed every time the piston rises in the cylinder, in order to assist the 

 discharge of the foul air. 



If, at the commencement of the operation, there is reason to believe 

 that the lungs are too heavily charged with foul air, the elastic Indian- 

 , rubber tubes ought alone to be brought into the nose, and, while keeping 

 the mouth air-tight with the hand, a few strokes will immediately remove 

 and discharge it. This operation will give room immediately for the fresh 

 air to act with success by means of the pipe o, which must then be put into 

 the mouth. Should it be wished to make an experiment to draw the foul 

 air out by the mouth, and to introduce the fresh air through the nose, un- 

 screw the pipe o and the tubes /, then turn the pump, so that the cylinder 

 b is up, and a downwards, the pipes and tubes are then replaced, so that I 

 is joined to the cylinder b, and o to a. 



The valves are placed in such a manner between the screws at the bot- 

 tom of the cylinder, that they may easily be taken out and turned an- 

 other way, as is shown in the drawing, so that they have different direc- 

 tions, and may act in either way, as is judged necessary. If it be wished 

 to make a trial of purified air, the apparatus containing the air must be 

 screwed into the opening k of the introduction pipe i, the gas will imme- 

 diately, by drawing the handle of the pumps upwards, float out of the ap- 

 paratus into the cylinder a, and in consequence, by the re-action of the 

 pistons downwards, be introduced into the lungs ; and should the room, 

 where the operation is performed, be too close, and filled by foul air, then 

 take a long tube, and place the funnel either out of the window, or into the 

 nest room, where the air is cool and fresh. 



9. Professor Amici' s Improved Camera Lucidas. 

 In using the ingenious camera lucida, invented by Dr Wollaston, and de- 

 scribed in several English works,* a practical difficulty has been experienc- 

 ed arising from the alternate appearance and disappearance of the point of 

 the pencil by which the outline is traced. In order to understand this, says 

 Professor Amici, let ABCD, Plate I. Fig. 15, be Dr Wollaston's quadrangu- 

 lar prism. The eye at O, perceives by means of two reflections from DC, CA, 

 the object Q, and refers it to P. The pencil held in the hand at P is seen by 

 one-half of the pupil, and the object by the other half, so that, by a slight 

 motion of the eye, the pencil, or the ray, is seen indistinctly, according 

 as the part of the pupil by which they are viewed becomes greater or 

 smaller. In order to avoid this evil, M. Amici adopted the construction in 

 Plate I. Fig. 16, where ABC is a metallic mirror, whose polished surface 

 AB, is inclined 135' to the plain surface BD, of a piece of glass DCFE, 

 with parallel faces. The eye at O, now sees the object R and Q, by the 

 rays RMPO, reflected at M and P. As both the pencil and the rays are seen 

 with the whole pupil, the object may be drawn with the greatest facility. 

 When lenses are used in this construction, a concave one should be placed 

 before the mirror ABC, and when a convex one is used, it should be placed 



" See the Edinburgh Encyclopedia, Art. Camera Lucm.v. 



