ABBE'S CAMERA LUCIDA 



28l 



on the paper the outlines and details of the image with ease and 

 accuracy ; only it must be remembered that the mirror or prism 

 erects the inverted image (No. 1 in the above table), but its trans- 

 position is due to the fact of its not being viewed as a transparency. 



This instrument is also useful for the purpose of demonstrating 

 where two or three persons may at the same time examine the 

 image, and it can be used on many opaque objects, and objects pre- 

 sented by dark ground illumination ; but to use it the external light 

 must be carefully screened from the observer. 



Coming now to the second group of cameras, there stands first on 

 the list an instrument devised by Professor Abbe ; although, like 

 many ' new ' apparatus for the microscope, the idea it embodies is 

 not a new one, but was suggested for micrometric purposes by Mr. 

 ft. Burch in 1878 (Journ. Quek. Micro. Club, v. p. 47). We have 

 used this admirable instrument with complete success. 



The accompanying drawing (fig. 220) will at once show the 

 simplicity of its action. The image of the paper and pencil coming, 

 say, in a vertical direction (S 9 fig. 220), is reflected by a large mirror 



w 



FIG. 220. Abbe's camera lucida. 



in a horizontal direction, W, to a cube of glass which has a silvered 

 diagonal plane with a small circular hole in it in the visual point of 

 the eye-piece. The microscopic image is seen directly through this 

 aperture in the silvering of the prism, while the silvered plane of 

 the prism transmits the image of the paper and the operator's fingers 

 and pencil. By the concentricity thus obtained of the bundle of 

 rays reaching the eye from both the microscope and the paper, the 

 image and the pencil with which it is to be drawn are seen coinci- 

 dentally without any straining of the eyes. 



This instrument requires the paper to be placed in a plane 

 parallel to that of the object ; thus, if the microscope is vertical the 

 paper must be horizontal, and vice versa, and it presents the image 

 precisely as it is seen in the microscope. For the purpose of drawing 

 simply, and where the observer has had no experience in the use of 

 a camera lucida, we should be inclined to recommend this one as the 

 instrument presenting to the tyro the greatest facility. But there 

 is a use to be made of the camera lucida to which this one does not so 

 readily lend itself, which is none the less of great importance ; that is, 



