A few months ago, news from Europe reached this country that another and seemingly more successful attempt had been made to produce glass that would leave no secondary spectrum, and that Dr. Zeiss, the famous Jena optician, had constructed some new improved objectives from it.  But the somewhat meager description of these objectives, as given by an English microscopist, did not seem fit to excite much enthusiasm here as to their superiority over what had already been done in this country.  Besides this, the report said that the new objectives were five system, and also that extra eye-pieces had to be used with them.  I confess I am much inclined to attribute the optical improvement, which, according to Dr. Abbe’s own remark, is very little, more to the fact that the objectives are five system than to the new glass used in their construction.

After a close study of a descriptive list of the new glass, received a week or two ago from the manufacturers, I find, to my great regret, that this new glass seems to suffer from a similar weakness to that made by the English firm twelve years ago; as all the numbers of the list pointed out by the makers as having a greatly reduced secondary spectrum are accompanied with the special remark “to be protected.”  Furthermore, from a comparison of the dispersive and refractive powers of these glasses, as given in the list, I find that objectives constructed from them will leave so great aberrations of higher order, both spherical and chromatic, that the gain by the reduction of the secondary spectrum would be greatly overbalanced.

In conclusion, I wish to say that while I would beware of underestimating the great scientific and practical value of the endeavor of the new German glass makers to produce improved optical glass, and the great benefit accruing to opticians and all others interested in the use of optical instruments, I think it wise not to overestimate the real value of the defects of the common crown and flint glass, which I have sought to explain in this paper.  And, for myself, I prefer to fight the more serious defects first, and when its time has come I will see what can be done with the secondary spectrum.

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PROBABLE ISOLATION OF FLUORINE.  DECOMPOSITION OF HYDROFLUORIC ACID BY AN ELECTRIC CURRENT.

By M.H.  MOISSAN.

In a former memoir[1] we showed that it was possible to decompose anhydrous hydrofluoric acid by the action of an electric current.  At the negative pole hydrogen collects; at the positive pole a gaseous body is disengaged, having novel properties.  The experiment was performed in a platinum U tube, closed by stoppers of fluorite, and having at the upper part of each branch a small delivery tube, also of platinum.  Through the stopper passes a platinum rod, which acts as electrode.  The metal employed for the positive pole is an alloy containing 10 per cent. of iridium.