Page:Popular Science Monthly Volume 60.djvu/142

134 lines of the spectrum could be combined. The spectrum outstanding was a tertiary one and much less marked than that due to the original crown and flint glass. The modern microscope became possible.

The conditions to be satisfied in a photographic lens differ from those required for a microscope. Von Seidel had shown that with the ordinary flint and crown glasses the conditions for achromatism and for flatness of field cannot be simultaneously satisfied. To do this we need a glass of high refractive index and low dispersive power or vice versa; in ordinary glasses these two properties rise and fall together. Thus crown glass has a refractive index of 1.518 and a dispersive power of .01 66, while for flint the figures are 1.717 and .0339. By introducing barium into the crown glass a change is produced in this respect. For barium crown the refractive index is greater and the dispersive power less than for soft crown. With two such glasses then the field can be achromatic and flat. The wonderful results obtained by Dallmeyer and Boss in this country, by Zeiss and Steinheil in Germany, are due to the use of new glasses. They have also been applied with marked success to the manufacture of the object glasses of large telescopes.

But the Jena glasses have other uses besides optical. "About twenty years ago"—the quotation is from the catalogue of the German Exhibition—"the manufacture of thermometers had come to a dead stop in Germany, thermometers being then invested with a defect, their liability to periodic changes, which seriously endangered German manufacture. Comprehensive investigations were then carried out by the Normal Aichungs Commission, the Reichsanstalt, and the Jena glass works, and much labor brought the desired reward." The defect referred to was the temporary depression of the ice point which takes place in all thermometers after heating. Let the ice point of a thermometer be observed; then raise the thermometer to say 100° and again observe the ice point as soon as possible afterwards; it will be depressed below its previous position; in some instruments of Thuringian glass a depression of as much as 0°.65 C. had been noted. For scientific purposes such an instrument is quite untrustworthy. If it be kept at say 15° and then immersed in a bath at 30° it will be appreciably different from that which would be given if it were first raised to say 50°, allowed to cool quickly just below 30°, and then put into the bath. This was the defect which the investigators set themselves to cure.