ACCESSORY INSTRUMENTS. 145 



be effected. The chief difficulty in the use of this instru- 

 ment is that of the artist being able to see at one and the 

 same time the pencil and the image ; to facilitate this in some 

 measure, Mr- Ross places either one or two lenses below the 

 prism, in order that the rays from the paper and pencil may 

 diverge at the same angle as those received from the prism, 

 whereby both object and pencil may be seen with the same 

 degree of distinctness. 



Messrs. Powell and Lealand supply with their microscopes 

 a small highly-polished steel mirror, fixed at an angle of forty- 

 five degrees, and placed in front of the eye-piece, where 

 it is held by a spring cHp, as represented by fig. 100. This 



mirror, being smaller than the 

 pupil of the eye, allows the rays 

 of light from the paper to enter 

 the eye around it, so that both 

 the paper and the image reflected 

 on it by the mirror, may be seen 

 at the same time and under the 

 same angle. A good form of 

 camera lucida, constructed by M. 

 Fig. 100. Nachet, together with other in- 



struments of a similar nature, will be described in the chapter 

 devoted to the uses of the camera in drawing and in micrometry. 

 Indicator. — For the purpose of pointing out to those who 

 are uninitiated in microscopic research any particular part of 

 an object that may be in the field of view, various contrivances 

 have been had recourse to ; but the author, who has often 

 found the want of some kind of indicator, first employed a 

 slip of glass, on which were ruled two lines at right angles 

 to each other. This slip of glass was mounted in a frame 

 of brass, and, like the micrometers of Mr. Jackson, here- 

 after to be described, was slid in through an oblong opening 

 in one of the eye-pieces in the focus of the eye-glass ; by the 

 ruled glass the field of view was divided into four compart- 

 ments, and any object therein could be so arranged by the 

 adjustable stage, that it might be in the first, second, or any 

 other compartment, or even be so placed, that the lines 

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