NACHET, ON THE CAMERA LUCIDA. 157 



this form of instrument 

 arise from the nature 

 itself of the mirror, its 

 easy destruction, and 

 the loss of light. 



Another instrument 

 of the same class, and 

 which affords very 

 beautiful effects, is the 

 camera of Doyere, 

 which is formed of a 

 very small prism placed 

 above the ocular, and 



of the reflecting prism of Amici's camera. Its inconveniences 

 are a certain delicacy of construction, and the difficulty 

 of using it ; or as observed by Mr. Gray, it is rather difficult 

 to adjust the eye to the angle of a prism. 



Nobert conceived the idea of replacing Amici's mirror by an 

 inclined slip of glass ; but with this it is difficult to equalize 

 the light between the field and the paper, and the two sur- 

 faces of the glass give a double image of the pencil. 



We now come to the second class of cameras, or to those 

 which give the image of the object by reflexion. These, 

 after mature consideration, we are of opinion should be 

 wholly abandoned. They have the grave inconvenience of 

 reversing the image, so that when it is wished to retouch the 

 drawing by the eye after the first sketch, it is requi- 

 site to allow for the reversal of the image, without which, 

 the draughtsman is always liable to place what should be 

 above below, and the reverse. Errors of this kind are 

 especially easy to make when the body designed has a 

 rounded or symmetrical contour, — such as embryonic 

 cells, &c. \Vollaston's prism, which belongs to the present 

 class, affords, it is true, an image in the same position as that 

 in the microscope, but most often it does not take in the 

 whole field ; and besides, in order "to use it, the microscope 

 must be nearly horizontal. 



The problem to be solved therefore being, that it is required 

 to see first the object, and indirectly the pencil, we have re- 

 solved it in the following manner. For the vertical microscope 

 we have adopted the form already described in Dr. Carpenter's 

 work on the microscope. The objections made to its use 

 by Mr. Gray arise from his not having employed it. Thus, 

 with respect to the sensation produced by the appearance of 

 the hand in the field of vision, there is nothing disagreeable 

 or annoying in this ; but, on the contrary, the motions of the 



VOL. VIII. o 



