Proceedings of the Polytechnic Association. 1051 



ableness of color, transparency, and a certain degree of refractive 

 power may be obtained, but perfect uniformity in the structure of the 

 glass, so as to render its composition absolutely homogeneous in all its 

 parts, is not so easy to be accomplished, and it is precisely this quality 

 which is the most indispensable in the manufacture of optical glass. 

 The achromatic telescope has been of the utmost importance in the 

 science of astronomy. Galileo, Dolland, D'Artigus, Guinaud, Utz- 

 schneider, Bontemps and Ross have all contributed to accomplish 

 the object ; and Frauenhofer and Fresnel have carried off the palm in 

 the solution of these great problems. The telescope and microscope 

 of 1869 are proofs of what has been done in this department of 

 applied science. 



At the close of the doctor's paper, several inquiries were made by 

 persons in the audience. 



Mr. Fisher wished to know Dr. Feuchtwanger's opinion in regard 

 to water in pipe made of coarse bottle glass. The doctor replied 

 that water pipes of this kind had been largely used in England 

 and to a limited extent in this country. Mr. Hamilton E. Towle 

 presented a specimen of what is known as " black diamond," such 

 as are used for drills, for cutting rocks and millstones and wished to 

 know if they were real diamonds or not. 



Dr. Feuchtwanger. — I presume they are selected sj^ecimens of 

 corundum. I never saw more that two specimens of real black 

 diainond in my life. 



Dr. Yanderweyde thought, from a slight examination, that they 

 were true diamonds. There are specimens of diamond which are 

 dark and low-priced, and may be used for drills and picks. 



Hill's Air Alarm. 



Mr. Hamilton E. Towle presented a model of a Patent Air Alarm, 

 invented by John O. Hill, of New Hampshire, for giving notice of 

 the attack of burglars upon banks, safes, dwellings, &c., and for the 

 purpose of preventing the escape of prisoners from jails and peni- 

 tentiaries, etc. Tlie principle of the invention consists in using tubes 

 either charged with air or exhausted. These tubes are connected 

 witli conducting pipes. 



Any attack whatever upon the pipe, tending to derange it, com- 

 municates to the alarm clock any change in the density of the air 

 enclosed in the pipes. The alarm clock may be situated at any 

 desired point, and every interference with the pipe will cause a 



