158 



NACI1ET, ON THE CAMERA LUCIDA. 



pencil acquire a resolution and firmness rarely met with in 

 the use of this kind of instrument. Some precautions should 

 be taken in the construction. The angle of the small at- 

 tached prism, e, should he 

 equal to the angle a, so that 

 the surfaces presented to the 

 eye are well parallel. The 

 surface b should be slightly 

 inclined, so that the pencil 

 may be seen at some dis- 

 tance from the instrument, 

 in order not to come in con- 

 tact with the stage. The pro- 

 jection of the image is thus 

 rendered a little oblique, 

 but to so very trifling an 

 extent that the deformity is 

 almost inappreciable ; and besides, if the paper be a little 

 inclined, this defect is removed. 



The arrangement adopted for the inclined microscope is 

 based on the same principle as the foregoing. A prism 



giving two total reflexions 

 is placed above the ocular, 

 so that the rays coming 

 perpendicularly from the 

 table are reflected by d, c, 

 and then by c, d, so as to 

 issue by the same surface, 

 b, c, which is at the same 

 time a surface of reflexion 

 and of transmission. An 

 image is thus afforded of 

 the object viewed by the 

 small prism e, whilst that of the pencil comes from c, d. It 

 follows that, by this prism, 1st, the image of the pencil 

 appears to be transported into the field of view, and that the 

 object is viewed in its true situation; and 2dly, that the 

 observer, looking into the ocular, enjoys the advantage of the 

 comfortable inclination of the instrument, whilst in the other 

 systems of cameras the inclination of the microscope is with- 

 out advantage, because it is necessary to look vertically upon 

 the table. 



