SCIENCE. 



203 



the discoveries rest are conflicting among themselves, and 

 if the probability of such discoveries is rendered small 

 by long and careful series of independent observations, 

 we are justified in waiting for further evidence before we 

 accept the alleged discoveries as true. The ways in 

 which an observer may be deceived are numerous. In 

 1878 an astronomer wrote me that he had discovered a 

 satellite of Venus that revolved around the planet in 

 thirty seconds. I expressed some doubt and advrsed 

 him to examine his telescope and the eye pieces. He did 

 so and was candid enough to inform me that the satellite 

 he had discovered was nothing but a " ghost." 



If any astronomer who is familar with astronomical 

 observations and their discussion, will examine the re- 

 ports on the Vulcans supposed to have been discovered 

 during the eclipse of 1878, and will notice how the re- 

 ports were changed from time to time, he will find good 

 reasons for doubt. Certainly this matter is not to be 

 settled by assertion. If there are Vulcans of the fourth 

 and fifth magnitudes which attain an angular distance of 

 from three to seven degrees from the sun, they ought to 

 be found easily. A. Hall. 



Washington, April 25, 1881. 



THE SOLAR PARALLAX. 

 To the Editor of " Science " : 



From the American (photographs of the Transit of 

 Venus, as presented in part the first of " Observations of 

 the Transit of Venus, December 8, 9, 1874, Made and 

 Reduced Under the Direction of the Commission Created 

 by Congress," I have obtained, for the value of the solar 

 parallax, 8.883" ± °-°34" corresponding to a distance be- 

 tween the centres of the sun and the earth equal to 

 92,028,000 miles. D. P. Todd. 



Washington, April 26, 1881. 



ASTRONOMY. 

 Mount Etna Obsesvatory.— The Memoirs of the 

 Italian Spectroscope Society contains an illustration of 

 the Observatory of Mount Etna, showing that work upon 

 the building has progressed as rapidly as could have 

 been expected, when we consider the difficulties to be 

 overcome in the transportation of materials, etc. Every 

 effort is being made to finish the Observatory by 1882, 

 and provide rt with a director and staff both of astrono- 

 mers and meteorologists. W. C. W. 



Messrs. Houzetu and Lancaster, the Director and Li- 

 brarian of the Bruxelles Observatory, are performing an 

 extremely valuable service to astronomers by the prepara- 

 ration of a general bibliography of Astronomy. Two 

 v olumes have thus far appeared, the second of which is 

 just published, and is devoted to memoirs which have ap- 

 peared in scientific periodicals, and in the publications of 

 the various academies. Four topics are included in this 

 volume, Spherical Astronomy, Theoretical Astronomy, 

 Celestial Mechanics, and Physical Astronomy. The only 

 thing which even approximates the completeness of the 

 present work, is the catalogue of the library of the Poulk- 

 ova Observatory, a new edition of which has been in 

 course of preparation for several years past. The Brux- 

 elles work, however, has the advantage of being a general 

 bibliography, and not limited to the contents of any one 

 library, however extensive. O. S. 



MICROSCOPICAL NOTES. 



At our suggestion, Mr. Lockwood, of New York City, 

 who has already devoted considerable attention to the 

 application of Photography to the various branches of 

 science, now proposes to make arrangements for ph Ho- 

 graphing Microscopical Preparations. 



The objects will be enlarged by very perfect and pow- 

 erful objectives, and photographed while thus enlarged. 



Those possessing microscopes will at once notice the 

 great advantage to be secured by such an arrangement. 

 Few possess the skill to produce a drawing from a 

 microscopic object, while the amount of detail involved 

 in sketching anatomical preparations, can be mastered by 

 few who are not professed artists. 



When Mr. Lockwood's arrangements are complete a 

 microscopist, for a moderate amount, will be enabled to 

 have a perfect copy of any microscopic preparation, and 

 as many duplicate as he requires to circulate among spec- 

 ialists, or his friends. Should he desire to publish the 

 result of his researches, Mr. Lockwood can then photo- 

 graph the object directly on the wood block, ready for 

 the hand of the engraver. 



The chief value of the use of Photography in such a 

 case lies in the fact that such drawings, being prepared 

 by the hand of nature, their integrity cannot be im- 

 peached, and that any charge of exaggeration or error 

 cannot be maintained. 



When Mr. Lockwood's arrangements are complete we 

 will announce the fact in our microscopical column, but 

 in the interval would be glad to hear from those who are 

 likely to avail themselves of these facilities for promot- 

 ing microscopical research. 



NOTES. 



Les Mondes proposes to apply rhe • photophone to the 

 study of the aurora borealis. 



On the Galvanic Polarization produced by Metallic 

 Deposits. — The polarization of copper, employed as nega- 

 tive electrode in a solution of sulphate of zinc, is never 

 null, as Lipmann believes, in cases where the solution con- 

 tains traces of a salt of copper, and that the deposit of 

 zinc is exceedingly slight and invisible. On the contrary, 

 it has a value which may differ much, and which is so much 

 the greater the smaller the quantity of a copper-salt con- 

 tained in the solution, and the less the time which has 

 passed from the moment when the polarizing current was 

 interrupted. — D. Macaluso. 



On the Electromotive Force of Voltaic Arc. — When 

 an electric flux is established between two conductors of 

 the same nature by means of a gaseous medium, which is 

 commonly the vapor thrown off by their substance, the in- 

 equality of temperature of those portions of the conductors 

 which are contiguous to such a medium appears to be a 

 general fact. It seems not less probable that the extremity 

 by which the positive electricity arrives, possesses the 

 higher temperature. This is observed in a remarkable de- 

 gree in the production of the voltaic arc between two car- 

 bons, by means of a current of constant direction. The idea 

 of ascribing to this phenomenon a thermo-electric origin 

 is not novel. According to the application of the principle 

 of the equivalence of heat to electric phenomena, an elec- 

 tromotive force acting in the inverse direction of the cur- 

 rent, corresponds to a disengagement of heat at the point of 

 junction. of two heterogeneous substances. — M. F. P. Le 

 Roux. 



Magnetic Action upon the Fluorescent Light pro- 

 duced by the Negative Discharge in an Exhausted 

 Space. — If we take a well-exhausted cylindrical tube, with 

 rectilinear electrodes placed in its axis, the fluorescent light 

 formed by the cathodic ravs consists, as is well known, of 

 a green cylinder bounded bya circle. Thiscircle undergoes 

 transpositions if a magnet is allowed to act upon the dis- 

 charge. It can be shown that these, whether simple or com- 

 plicated cases, may be explained by the following hypothe- 

 sis: — The cathodic rays, emanating from the negative 

 electrode, pass on in a straight direction, and the current 

 moves from the anode to the sides of the cathodic space, 

 and from thence to the negative electrode. The magnet 

 acts upon these currents according to Ampere's rule. — K. 

 Domalip. 



