The Philosophical Instruments in the Paris Exhibition. 427 



however, exhibited by Secretary of Paris, was in all respects a 

 most excellent instrument. The chief interest consisted in the 

 speculum, which was one of Foucault's. 



Instruments made upon this principle by English artists 

 are so well known to readers of The Intellectual Observer, 

 that I need not dilate upon their merits, or explain their con- 

 struction. 



The display of mounted and hand telescopes for military, 

 naval, and travelling purposes, was extremely large and excel- 

 lent. The most important novelty in these instruments was to 

 be seen in those exhibited by T. Ross and Dallmeyer. Their 

 focal length is only from nine to ten times the diameter of the 

 clear aperture ; notwithstanding which, they combine excellent 

 definition with great portability. Messrs. Beck exhibited new 

 and very ingeniously constructed binocular telescopes with which 

 some extremely interesting features connected with binocular 

 vision can be examined. A vast variety of binocular opera- 

 glasses were exhibited by various makers. The low prices of 

 M. Bardou's small telescopes and opera-glasses considered in 

 connection with their excellence, is astonishing. He manufac- 

 tures enormous quantities of these instruments for England, 

 where they obtain a ready sale. 



France and Prussia made an extremely fine display of alt- 

 azimuth, meridian circles, and transit instruments. Those 

 exhibited by Brunner of Paris were highly remarkable for 

 combining the great desiderata of lightness and rigidity, and 

 for being applicable either to astronomical or geodisical pur- 

 poses, both circles being read off by means of micrometer 

 microscopes. The graduation of these instruments was excel- 

 lent ; instead of the figures being cut on brass or silver, they 

 are white on a black ground, and can be read with very great ease. 



To no new discovery are philosophical instrument makers 

 more indebted than to aluminium. This light and beautiful 

 metal is peculiarly suited for astronomical instruments. Besides 

 its great lightness, it is extremely rigid, and but little liable to 

 tarnish. Many instruments in the Exhibition were made of 

 this metal, and particularly of bronze aluminium. This is com- 

 posed of ten parts of ordinary aluminium, and five of copper, 

 and it possesses a strength both vertical and transverse far 

 exceeding that of wrought iron. This metal is especially valu- 

 able in the manufacture of sextants, which require to be held in 

 the hand, and often for a long time. Bearing in mind how 

 highly important a part the sextant plays in navigation, it is 

 lamentable how worthless a large proportion of those instru- 

 ments are. Happily, however, great pains have been lately 

 taken by philosophical instrument makers to improve them, and 

 with considerable success. The show of sextants at the late 



