290 Miscellaneous Intelligence. 
~~ 
characteristic Mexican indolence could not be overcome. The only 
answer [ could obtain, was the metal was “ muy duro.” 
ese pieces were brought from a valley in a small mountain chain, 
about forty miles southeast of Tucsan, east of the road leading to 
Tuvaca. In this valley, fragments similar to those seen, and of various 
sizes were said to be abundant. From the occurrence of this metal, the 
valley was called “Cafiada de Hierro,” or iron valley. Silver mines 
of great richness are very numerous in that vicinity : the metal occurs 
as sulphuret, with galena and blende, and also in the native form. 
of it on account of its scientific import It a clear 
concise statement of the difficulties and perplexities that attended 
those wonderful improvements in the manu ur , recently 
made by him; and, on account of which, it will be remembered t 
he. Committee appointed at the Albany meeting of the American Asso- 
ciation for the Advancement of Science, held in August last, decided 
that his glasses are quite superior to any now made in the world. Oa 
this account the letter will be read with interest by all, and especially 
by those who are versed in the optics of Microscopy. i 
explain briefly the defects of the best objectives at the time I com- 
, W 
powers. ‘The objectives of Pritchard, Chevalier Oberhauser, Pleesse 
and Schieke, rarely reach this number of degrees, even at the present 
time. Owing to the character of*the materials at the command of the 
working opticians, in connection with the fact, that all the existing 
works upon science were silent in reference to any formula or laws of 
combination, the construction of objectives had become nearly station- 
ary as regarded their most 
_ The method then employed by the best artists was one entirely prac 
_tical ; and consisted merely in combining three double achromatics; 
jective ; and the differences which were to be found in the objectives 
of different artists, consisted mainly in the focal lengths employed 
