104 



Correspondence of J. Nickles. 



Porro has invented a very simple machine by means of which a spher- 

 ical surface of given radius may be cut, " sans bassins," and then this 

 radius may be varied by insensible degrees with great perfection. This 

 method, in connection with another piece of apparatus of his invention, 

 the poly optometer — for the examinations which should precede the cut- 

 ting, and a new application of Frisiani r s method in what pertains to the 

 verification of the work at every step of advancement, — allows of arriving, . 

 without consulting the heavens, so near to perfection, that little remains 

 to be done for attaining all desired distinctness. 



Equatorial Telescope. — This instrument, also made by M. Porro, is 

 equal in dimensions and power to that which has long been called the 

 Colossus of Dorpat, but it displays great simplicity and has several pecu- 

 liarities. The rotations of this instrument are spherical, and the trans- 

 mission of the diurnal movement is made by the adhesion of two spher- 

 ical surfaces. There is no window counterpoise ; but lubricating oil 

 introduced together with pressure is advantageously substituted. The 

 clock movement is produced by a hydraulic motor of peculiar construc- 

 tion, the arrangements of which are simple and convenient. These com- 

 binations are all such as to avoid the defects of wear. 



Zenith Telescope. — There is also, just now, at M. Porro's, a zenith tel- 

 escope, bought by an amateur astronomer. It is eighteen decimeters 

 long and has an aperture of one decimeter. This instrument gives at 

 any time without inversion and without level, the exact place of the ze- 

 nith. The latitude and time may hence be determined by means of it 

 with the greatest precision and in the briefest time. M. Porro calls this 

 telescope the direct zenith tube in distinction from the reflex zenith tube of 

 Mr. Airy, which gives the zenith by reflexion upon a mercury bath. 



Stereoscopic experiment. — M. Lugeol, " Contre-Amiral," in making the 

 stereoscopic portrait of one of his friends, had the idea of taking the two 

 images or proofs one after another, and making his friend each time look 

 upon a different object. Thus during the first sitting he looked at the 

 object glass of the camera obscura, and during the second to the right 

 at an object fixed nearly at an angle of 45°. These two images being 

 placed in the stereoscope, let the observer stand opposite a window and 

 without ceasing to look at the portrait, turn himself to the left or right, 

 he will see the eyes of the portrait follow him as if they were animated. 



Use of brine in food. — In consequence of accidents caused by the use of 

 the brine of herring or salt meat, the council of health in Paris has been 

 charged with examining to what extent brine may be allowed in food. 

 Numerous experiments have been tried at Alfort, which have led to the 

 following conclusions. 



" The use of brine as a condiment or seasoning in the nourishment of 

 man has hitherto had no injurious effect, and nothing authorizes the opin- 

 ion that an economical process so advantageous for the poor should be 

 proscribed. The same is not true of the abuse which is made of this sub- 

 stance in the nourishment and in the treatment of the diseases of certain 

 animals, especially swine and horses. Authentic facts and recent experi- 

 ments show that the mixture of brine in considerable quantity with food 

 may produce real poisoning. In all cases, brine preserved too long or in 

 contact with rancid meat should not be employed except with the great- 

 est care and after it has been purified by skimming all the scum which 

 forms on the surface." 



