225 



NA TURE 



\Jan. 17, 1878 



piece, just three times ttie quantity of homogeneous glass 

 that the Vienna discs required, and though I am not of 

 course in the secrets of the glass manufacturers, it 

 appears to me that the chances of obtaining 40-inch discs 

 in the present state of the art are remote. 



" The other difficulties of manufacture of refractors 

 consist in the nicety of the operation connected with the 

 calculations of the curves, the manipulation of such 

 extremely costly material, and the enormous labour and 

 trouble of the figuring and perfecting of the objective. 

 All these, however, I have no doubt will be overcome by 

 the optician for any size which the glass-maker is at all 

 likely to produce. 



" Now, as to the difficulties connected with the manu- 

 facture of reflectors, whether metallic or silver on glass. 



" First, as to the difficulty of producing the metallic or 

 glass disc to work upon. 



" Lord Rosse has succeeded years since in casting, 

 annealing, and perfecting discs of six feet in diameter, 

 and any difficulties he met with were not such as to lead 

 me to the belief that the limit of possible size has been by 

 any means reached. As regards glass mirrors, the ques- 

 tion has never been discussed, for in any sizes that have 

 been made up to the present time, it was only necessary 

 to go to the plate-glass manufacturers and say, * I want a 

 disc of crown glass of such a diameter and such a thick- 

 ness,' and forthwith the glass disc was delivered without 

 any trouble ; but, when we come to these extraordinary 

 sizes, it is quite a different matter. For the 4-foot disc 

 of glass for the Paris reflector, in place of that which has 

 so recently resulted in failure, the St. Gobain Glass Com- 

 pany require twelve months' time to perfect (although, be 

 it remembered, the quality of the glass is here of no 

 consequence whatever) ; and I have been myself in corre- 

 spondence with the principal glass manufacturers here 

 and on the Continent, and not one of them is willing to 

 undertake even a 6-foot glass disc ; so that it would 

 appear that, above that size, the silver-on-glass mirrors 

 are out of the question. 



" This much, however, is to be said : If anyone were 

 to go to a brass- or bell-founder's and ask them to under- 

 take a speculum of six feet in diameter, he would 

 almost certainly be met with a refusal ; and yet Lord 

 Rosse has proved the feasibility of it. And so, reasoning 

 by analogy, might the manufacture of a six- or eight-foot 

 glass mirror be possible, if undertaken in the same scien- 

 tific spirit in which Lord Rosse undertook his. I answer to 

 this — Yes ; perfectly true ; but this is too purely a specu- 

 lative matter to be considered at the present day in the 

 choice of telescopes. 



" The other great difficulty in the manufacture of reflec- 

 tors is the anneialing of the disc, and I believe it is this 

 difficulty which limits to so narrow an extent the produc- 

 tion of glass discs for silver-on-glass mirrors." 



Vv'^e can abundantly gather from this paper of Mr. 

 Grubb's that our opticians are doing all that lies in their 

 power to give us increased power in the future. The fact 

 that in the last few years one refractor of 25 inches, and 

 two of 26 inches, have been acquired to science, leads us 

 to hope that for the present progress will lie in increasing 

 the dimensions of that instrument. Mr. Grubb, indeed, 

 has already in hand one of 27 inches for the Austrian 

 Government. The contretemps to the four-foot Foucault 

 in Paris will also help to set the tide in the same direction. 

 , From what has preceded it will be seen that each in- 

 crease in the power of the telescope is of little avail unless 

 we use it in purer and purer air. It is quite true that in 

 the telescope much of the injury to definition arising 

 from currents in the tube may be got rid of by the 

 employment of lattice-work ; but this, of course, will not 

 lessen the atmospheric effects of the column of air ever 

 increasing in diameter between the telescope and the 

 object. 



Prof. Piazzi Smyth's astronomical experiences on 



Teneriffe will still be in the minds of many of our 

 readers. He showed that an enormous advantage 

 was secured from observations so soon as half the 

 atmosphere was below the observer. A more recent 

 experiment by Dr. Draper, however, has shown that it 

 will not do to go blindly and put the telescope on any 

 high mountain. The conditions of each place from this 

 single point of view must be carefully studied. Summing 

 up his experiences of the Rocky Mountains up to heights 

 of 10,000 feet, Dr. Draper says : — 



" On the whole, it may be remarked of this mountain 

 region that the astronomical conditions, especially for 

 photographic researches, is unpromising. In only one place 

 were steadiness and transparency combined, and only 

 two nights out of fifteen at the best season of the year 

 were exceptionally fine. The transparency was almost 

 always much more marked than at the sea-level, but the 

 tremulousness was as great or even greater than near 

 New York. It is certain that during more than half the 

 year no work of a delicate character could be done. . . . 

 Apparently therefore, judging from present information, 

 it would not be judicious to move a large telescope and 

 physical observatory into these mountains with the hope 

 of doing continuous work under the most favourable 

 circumstances." J. Norman Lockyer 



{To be continued.) 



ELECTRICAL ANALOGIES WITH NATURAL 

 PHENOMENA 



WITHIN the last few years M. Gaston Plantd has at 

 intervals described a series of ve»y curious pheno- 

 mena produced by electric currents of high tension, and 

 has pointed out numerous analogies which they present 

 with several atmospheric and cosmical phenomena. With- 

 out committing ourselves to the belief that these analogies 

 are real, the phenomena described are so interesting 

 that we are glad to be able, by the kindness of M. Plantd, 

 to reproduce some illustrations of them. 



To obtain electric currents of high tension M. Plants 

 has employed secondary batteries of sheets of lead, 

 which, as is known, constitute powerful accumulators of 

 voltaic electricity. By associating a very great number 

 of batteries uniting from 400 to 800 of these secondary 

 couples, a discharge is obtained equivalent, according to 

 M. Plants, to that of from 600 to 1,200 Bunsen couples 

 arranged in tension. 



Fig. I represents the arrangement of 400 secondary 

 elements divided into ten batteries. This is the source of 

 electricity employed for some of the earlier experiments 

 which we are about to describe. The more recent ones 

 have been made with 800 secondary elements arranged in 

 twenty batteries of forty couples. A second series of bat- 

 teries similar to the first is arranged in another room, and 

 the curient which it furnishes is joined to that of the first 

 series by conducting wires suitably adjusted. These bat- 

 teries, associated at first in simple circuit by means of com- 

 mutators, do not require to be charged all at once like two 

 Grove or Bunsen couples. When they have not been out of 

 use for too long a time a few hours suffice to charge them. 

 We may then, by turning the commutators, unite all the 

 secondary elements in tension and use at will, either in a 

 few seconds or in a longer time, the enormous quantity of 

 electricity resulting from the chemical work accumulated 

 during two hours by Grove or Bunsen batteries. 



Such was the powerful means adopted by M. Plants in 

 making his late experiments. In his earlier experiments 

 he used a much simpler apparatus. 



The gyratory movements accompanied with luminous 

 effects which M. Plantd had observed with a powerful 

 current of electricity, and the spherical and annular forms 

 manifested by bodies submitted to that action, suggested 

 to M. Plantd the probability of the electric origin of the 

 forms of some of the nebulous masses of matter which 



