97 



Professor Bache called the attention of the Society to the 

 donation of transparent models of crystals, presented to the 

 Cabinet by Professor Alexander. 



He stated that these models had all the advantages of those made 

 from glass, with greater convenience in the construction of them. 

 The thin plates of mica are readily marked with a sharp instrument, 

 and easily cut. The parts are put together with diamond cement, it 

 having been found that this is a much better method of connecting the 

 pieces composing the model, than by cutting the sheets partly through 

 and using the mica as a hinge, which renders the sheets liable to 

 split. The forms resulting from the cleavage of crystals, &c, may 

 be represented in these models as in those of glass. 



Dr. Hays made a verbal communication relative to the 

 catoptric examination of the eye, as a means of distinguish- 

 ing the morbid conditions of the transparent tissues of that 

 organ. 



He stated that when a lighted candle is held before an eye, the 

 pupil of which is dilated, and in which there is no obscurity of the 

 transparent tissues, three distinct images of the flame is visible ; two 

 upright and one inverted, the latter appearing between the two 

 former. 



Experiments made to determine the causes of these reflected 

 images, and the changes which occur in their number, position, &c. 

 have shown that if a light be placed before the convex face of a single 

 watch glass, or of several of them superimposed, one or more up- 

 right images of the flame will be seen, according to the number of 

 glasses employed.* Now in the eye there are two superimposed 

 convex surfaces, viz. — 1st. the cornea; and 2d. the anterior capsule 

 of the crystalline lens. Thus the formation of the two upright images 

 is explained. Again, if a light be placed before the concave surface 

 of a watch glass, an inverted image is seen. Such a surface exists 

 in the eye, in the posterior capsule of the lens ; and thus the third 

 image is accounted for. 



M. Sanson, a distinguished French surgeon, has taken advantage 

 of the above facts, to distinguish cataract from amaurosis, and has 



* To be strictly accurate, it should be said that each of these images is 

 double, for one is reflected from each surface of the glass, and these images 

 are the more distinctly double, the thicker the glass. 



