414 D. J. SCOURFIELD ON ASHE's CAMERA LUCIDA. 



tint reflector. By making the mirror and the neutral tint glass 

 to rotate upon two parallel pins,* Mr. Ashe has succeeded in 

 producing a camera which can be used in any position of the 

 microscope. When the latter is inclined at any angle between 

 about 45° and the horizontal, the image can be projected (only 

 apparently, of course) vertically downwards on to the table by a 

 suitable adjustment of the relative positions of the mirror and 

 neutral tint glass. When the microscope is vertical (and also 

 when inclined) the image can be projected to the side by rotating 

 the camera 90° from its former position. The drawing-paper 

 must now be placed on a board inclined at the proper angle, i.e, 

 at right angles to the line of sight. 



I may mention in this connection that it is easy to know 

 when the drawing-board is correctly placed by observing the 

 outline of the image of the field. If this be a circle, the angle 

 of the board is correct. A sheet of paper with a series of con- 

 centric circles drawn upon it will enable the question of the 

 circular outline of the field to be settled quite readily. Such a 

 sheet of paper is also useful in determining whether the line of 

 sight is vertically downwards, as it should be, when the camera 

 is used, to project the image upon the table, as already described. 



There are one or two points in connection with the construction 

 of Ashe's camera lucida which it will be useful to mention. The 

 most important of these is that, if the camera is to be used not 

 only with low power, but also with moderately high power eye- 

 pieces, the mirror must be so arranged that its free end, when 

 the mirror is inclined at about 45°, is as close as possible to the 

 eye-lens. This can only be done by making the mirror rather 

 small, and pivoting it to the front plate just above the central 

 opening in the latter. 



* In the first model, only the neutral tint was movable, the mirror being 

 fixed at an angle of 45°. This fixity of the mirror prevented the object 

 under the microscope from being seen directly except by the removal of 

 the camera. In the form now described the mirror can be turned up so as 

 to allow of the object being viewed in the ordinary way, if required, without 

 detaching the camera. 



It may be interesting to note here that in 1894 Mr. Nelson, not knowing 

 what had been done by Mr. Ashe, brought forward a camera lucida (see 

 note " On a. new Camera. Lucida," Journ. Q. M. C, Vol. VI., p. 39), which 

 was practically the same as Mr. Ashe's original form, except that all the 

 parts were rigid, and that the mirror was arranged so as to reflect the 

 lig^ht on to a neutral tint glass placed at the side instead of below. 



