280 



ACCESSOKY APPARATUS 



the stage in the position as here printed ; when examined by the 

 microscope it will appear thus -J . In order to look at this letter as 

 the original, all that we have to do is to turn this paper round. 

 But this object, as drawn by a Beale's camera, will appear "^, and no 

 turning of the paper can cause it to appear as the original ; it will 

 only become so when it is viewed as a transparency from the other 

 side of the paper. 



This is, of course, important in many matters with which the 

 microscopic biologist is concerned. 



In many forms of camera this difficulty has been overcome by 

 reflecting the image of the paper and pencil down the tube of the 

 microscope. The drawing there made will be inverted and trans- 

 posed, but by turning the picture round we at once get a correct 

 representation of the object itself. 



The new camera devised by Mr. Nelson consists of a right-angled 

 prism or small glass mirror fixed at an angle of 45 to an eye-piece 

 cap. This, when the microscope is placed in a horizontal position, 

 reflects the rays horizontally arid at right angles to the optic axis ; 

 these rays then fall on a piece of neutral-tint glass placed at an 

 angle of 45 to those rays so as to reflect them upwards to the eye. 



The mirror corrects the transposition, and the neutral-tint the 

 inversion ; an erect image is therefore seen on the table. The neutral- 

 tint glass is mounted on a pivot so that it may be turned round at 

 a right angle ; this adapts the instrument for use with either the 

 right or left eye. Should the light be too strong, it must be 

 modified by screens, not by change of focus in the condenser, assum- 

 ing that the perfect image has been obtained. 



On the important subject of the inversion and transposition of 

 microscopic images brief but valuable data are given and put in the 

 clearest light, thus : 



Object on the 

 stage. 



F 



Image seen through 

 the eye-piece. 



Image projected on screen 

 or on sensitive plate. 



Image seen through Woll- 

 aston's camera. 



linage seen through 

 ground glass. 



F 

 6 



Image seen through 

 Beale's neutral tint or 

 Soenmieriug's reflector. 



Image seen through Nel- 

 son's camera. 



Image projected on. table 

 by 45 mirror or right- 

 angled prism, as devised 

 by C. W. Cooke. 



F 



The instrument referred to in (7) of the above table of inversion 

 and transposition in microscopic images is a somewhat distinct form 

 of camera called by Mr. Conrad W. Cooke, who devised it in 1865, a 

 ' Micrographic Camera.' The projection of the image is dependent 

 on a silvered mirror fixed at 45, or a right-angled prism. By the 

 arrangement of this instrument an image can be thrown on a sheet 

 of paper placed in a horizontal position, so that one can readily trace 



