134 POPULAR SCIENCE MONTHLY. 



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 simultane- 

 ously 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 flgures 

 are 1.717 and .0339. By introducing barium into the crown glass a 

 change is produced in this respect. For barium crown the refractive in- 

 dex is greater and the dispersive power less than for soft crown. With two 

 such glasses then the fleld 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 Ger- 

 man 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 endan- 

 gered German manufacture. Comprehensive investigations were then 

 carried out by the Normal Aichungs Commission, the Keichsan- 

 stalt, 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°, allow^ed to cool quickly just below 30°, and then 

 put into the bath. This was the defect which the investigators set 

 themselves to cure. 



DEPRESSION OF FREEZING POINT FOR VARIOUS 

 THERMOMETERS. 



Humboldt 1835 0°.06 



Greiner 1872 .38 



Schultzer 1875 .44 



