246 



NA rURE 



[July 12, 1900 



LETTERS TO THE EDITOR. 



: [ The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for tkii or any other part of Nature. 

 No notice is taken of anonymous communications. '\ 

 Eclipse Photography. 

 The writer has obtained results in photography which seem 

 to have an important bearing on the work which should be 

 undertaken in future eclipses. 



It is well known that photographic plates exposed in some 

 eclipses have developed no trace of an image of the sun. The 

 astronomer has even been subjected to the mortifying suggestion 

 that he had forgotten to uncap his camera. It is not difficult to 

 reproduce such results at any time by simple over-exposure. In 

 eclipse photography, where it is sought to get the most delicate 

 of details in an object of the most delicate character, the methods 

 now used are hedged in by very peculiar limitations. It requires 

 a very appreciable time to secure delicate details, and, never- 

 theless, if this time is made too great the plate will iog. The 

 developer must then be given restraining properties, which 

 cause a loss of the very details we are seeking to secure. 



In a paper recently published by the Academy of Science of 

 St. Louis, the writer has shown that a plate which, on account 

 of over-exposure will develop as a zero plate in a dark room, 

 will develop as a positive in a light room. The paper contains 

 a half-tone reproduction of a positive obtained by a camera 

 exposure of one minute, and developed within a few inches of 

 a i6-candle incandescent lamp. The plate was an "in- 

 stantaneous " Cramer plate. Since that time the same results 

 have been reached by first opening up the plate holder and 

 exposing the film to the lamp light until it is all converted into 

 the zero condition. If covered with an opaque punched stencil, 

 no trace of the design will appear on the film when developed 

 in the illuminated bath. The slide is then closed and the plate 

 afterwards exposed in (he camera in the usual way. Such a plate 

 cannot be over-exposed in any reasonable time. It may be 

 exposed for a minute or for four hours to a brilliantly-lighted 

 landscape, and the most superb results can be obtained. There 

 is no restraining developer needed. The tendency to fog when 

 the exposure is too short is corrected by taking the developing 

 bath nearer to the light. It seems probable that on very short 

 exposures it might sometimes be advantageous to use a developer 

 which will yield a positive with an under-exposed plate. In 

 the two eclipses of long totality which are now approaching, this 

 method seems to promise very valuable results, and the attention 

 of those who will have the work m charge is earnestly directed 

 to this matter. The results described have been reached but 

 recently', and there is need of preliminary experimenting by 

 any one who wishes to avail himself of these methods. 

 St. Louis, Missouri, U.S.A. Francis E. Nipher. 



The Action of Water Upon Glass. 



It is a matter of too frequent observation in India that lenses 

 of optical instruments are liable to serious injury from atmo- 

 spheric influences. This very often takes the form of injury to 

 the Canada balsam cementing the two lenses of achromatic 

 combinations together ; but in other cases, it is due to the 

 solvent action of water on the surface of the glass. As this 

 is a matter of importance necessitating the re-grinding of 

 the lens for its correction, I have thought that the following 

 observations may be of interest and of value to optical 

 instrument-makers, especially as it appears that only particular 

 kinds of glass are attacked in this way. If that is so, 

 it should be possible to avoid using glass of that particular 

 composition ; or the edges of the combined lenses may be 

 covered with a coating of cement or varnish so as to prevent 

 moisture getting in between them, and in such a way that it 

 could easily be removed when desired. 



My attention was first drawn to some cases of articles of 

 domestic glassware being attacked by water standing in them for 

 some time, and these are recorded to show that a solvent action 

 does take place. The first case that I noticed was that of a cut 

 wine glass which was used— or misused— to hold a few cut 

 flowers. On seeing it dry on one occasion, I noticed it had a 

 dull matt appearance, which I thought was simply a deposit. 

 On examination, however, I found that the surface of the glass 

 had been eaten into up to the level of the water usually put 

 into it. 



NO. 1602. VOL. 62] 



The next case was that of glass finger-bowls, in which the 

 servants kept water ready for use. These were similarly attacked 

 up to the level of the water. The next was a more remarkable 

 case. A couple of decanters, not required for use, had evidently 

 been washed and drained, more or less, but not dried ; possibly 

 during the hot season. The moisture remaining inside had 

 become deposited on the inner surface in droplets, as, indeed, 

 may frequently be seen, and had been standing so for some time. 

 When dried for use the surface was found to be eroded, giving a 

 pattern precisely similar to that formed by condensed moisture : 

 leaving no doubt as to its cause. 



Here we have, then, a case of pure water attacking the surface 

 of glass when allowed to stand for some time. Since then, being 

 on the alert, I have met other cases, including some of perfectly 

 new glass articles eroded in like manner, which, without their 

 history, it is impossible to account for. 



Now for two instances of physical apparatus being attacked 

 and spoiled by this action. The first noticed was a Newton's 

 Rings apparatus. In this case the two discs of glass were equally 

 attacked, and so much so that the combination was of a dense 

 matt appearance. On opening it out, the discs were found to be 

 firmly adhered, and on inserting a knife edge between the discs 

 and giving a sharp tap on the back to separate them, an irregular 

 piece about i^ inch long came from one adhering to the other. 

 The two had thus grown together, and at the junction was 

 actually stronger than in the mass of the glass. 



The next was a more serious case, being the object lens of a 

 3 J inch telescope from a well-known London firm of optical instru- 

 ment-makers. In this case, the convex lens was badly corroded on 

 its inner surface, though the adjacent face of the concave lens was 

 quite clear. Here we see the difference in action in the case of 

 two different kinds of glass. This, however, would help us little 

 if all kinds of crown glass (of which the convex lens is made) 

 were similarly attacked. But this is not the case, and it is a 

 point of importance to opticians to ascertain what particular 

 kinds of crown glass used in achromatic combinations are liable 

 to this action, and to avoid using them. Of a fairly large 

 number of achromatic combinations I have in the College 

 Laboratory, this is the only one that has been affected, though 

 all are exposed to the same influences, while some belong to old 

 pieces of apparatus. The particular telescope was purchased 

 about six years ago, and the damage took place in one season 

 when it was not much used. Since then I have from time to- 

 time opened out the lenses and have frequently found a layer of 

 moisture between them ; in one case, of a commoner piece of 

 apparatus in which the lenses did not fit closely, a complete 

 drop of water was collected, the diameter of the lens being only 

 \\ inch ; and in a Soleil's saccharimeter, clear through vision 

 is obscured by moisture collected and condensed on the surfaces 

 of the lenses in one of the adjusting pieces, which it is very 

 difficult to open out to clean. 



All this shows that moisture does collect in the form of water 

 between such layers of glass, and the pattern of the eroded 

 portion of the telescope lens, tosrether with the instances of the 

 action of water on the domestic glass goods mentioned above, 

 leave no doubt that it was moisture alone that caused the damage 

 in this case, although it was not actually seen. I need hardly 

 say that, in both the Newton's Rings apparatus and the telescope 

 lens, the exposed surfaces were perfectly clear and unacted 

 upon. 



The causes of moisture collecting in this way would appear to- 

 be the excessive moisture in the air for many months in the year, 

 the hygroscopic nature of the glass, and capillary action between 

 the surfaces ; while the apparently marked action of water on 

 glass here noticed is probably due to the long-continued higher 

 temperature. It is possible, however, that the above phenomena 

 may not be as new or unusual as they appear to be to me, and 

 that many others could give like experiences. 



Edmund F. Mondy. 



Dacca College, Dacca, East Bengal, June i6. 



THE TOTAL SOLAR ECLIPSE AS OBSERVED' 



B Y THE SMITHSONIAN EXPEDITION. 

 X^ADESBORO, in Northern Carolina, was the station 

 * * selected by the Smithsonian Institution for observ- 

 ing the total solar eclipse of May 28 last. The chances- 

 of fine weather at eclipse time were about eight to one,, 

 and it is satisfactory that on eclipse day the sky was- 

 cloudless and the air clearer than on the average. 



