218 PROCEEDINGS OF THE ACADEMY OP [1886. 



of the ligament is released. The lens is flattened, or its optical 

 axis is shortened, as soon as the ciliar}^ muscle is relaxed; the 

 ligament being drawn upon by the elasticity of the sclera, and 

 perhaps it is somewhat aided by the intraocular pressure. Dr. 

 Sharp stated that as far as he knew the mechanism of the 

 " lenticular expansion " had not been described. This action is 

 easily seen on turning to the development of the lens. The 

 lens is formed by an invagination of the external ectoderm soon 

 closing, and as a result we have a spherical vesicle, soon enclosed 

 in the mouth of the secondary optic vesicle. When this has 

 taken place, the posterior wall of the lens-vesicle thickens, that 

 is, the posterior cells commence to elongate, and grow toward 

 the anterior wall of the vesicle, the cells of which remain, 

 generally speaking, of the same size, and later form the so-called 

 epithelium of the lens. Keeping this structure in view, we see 

 that when pressure is brought to bear on the lens, these elongated 

 cells of the posterior wall are compressed in their longitudinal 

 axis, so that as soon as the pressure is removed, they simply 

 straighten out. This will also account for the fact, that the 

 anterior face of the lens is the onl}^ portion that moves in the 

 act of accommodation for distance. In the adult lens this 

 structure is to a certain extent lost, and the lens is generally 

 described as being made up of layers concentrically arranged. 

 This is true, but the embryonic " impression " still remains. If 

 we supposed that the lens were made up of layers concentrically 

 arranged and so formed^ when the capsular ligament " slacked 

 up " the tendency of the lens w^ould be to shorten its optical 

 axis instead of lengthening it. 



The Opal Mines of Queretaro^ Mexico. Dr. A. E. Foote 

 remarked that the localit}^ referred to is particularly interesting 

 as being the only one in North America that is being worked 

 solely for the production of gems. 



The opals of Mexico have been celebrated since 1820, when 

 Karsten and Del Rio referred to the opals of Zimapan and that 

 neighborhood as being in many respects equal and in some 

 respects superior to the Hungarian. There are quite a number 

 of fine localities from which considerable quantities have been 

 exported to Europe, so that among the lovers of the beautiful, 

 Mexico is as well known for its brilliant opals as for the soft and 

 exquisite tints of the tecali or Mexican onyx. 



As in the case of diamonds from Brazil and Africa, there is a 

 jeweler's prejudice against the new locality, and they are con- 

 sidered even more unluck}^ and liable to break than those of 

 Hungary, I have, however, seen in the possession of Senor 

 Cosio magnificent specimens that had been taken out over ten 

 years, and were without a flaw. 



The locality in Queretaro is the only one in Mexico that is 

 being worked to any extent now. 



