660 Mr. Bichard T. Glazebrooh [May 24, 



about 10 per cent., twenty-eight different elements, and the effect 

 of each of these on the refractive index and dispersion had been 

 measured. Thus, for example, the investigators found that by the 

 addition of boron the ratio of the length of the blue end of the 

 spectrum to that of the red was increased ; the addition of fluorine, 

 potassium or sodium produced the opposite result. 



Now in an ordinary achromatic lens of crown and flint, if the 

 total dispersion for the two be the same, then for the flint glass the 

 dispersion of the blue end is greater, that of the red less than for 

 the crown ; thus the image is not white : a secondary spectrum is 

 the result. 



Abbe showed, as Stokes and Harcourt had shown earlier, that by 

 combining a large proportion of boron with the flint, its dispersion 

 was made more nearly the same as that of the crown, while by 

 replacing the silicates in the crown glass by phosphates, a still 

 better result was obtained, and by the use of three glasses three 

 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. 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 Ross in this 

 country, by Zeiss and Steinheil in Germany, are due to the use of 

 these 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 

 Beichanstalt and the Jena Glass Works, and much labour 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 





