23 



When Dollond first commenced the manufacture of lenses 

 formed of two components, to correct the chromatic defects 

 of a single lens, there were available only two types of glass, 

 called respectively " crown " and " flint " glass, and these 

 glasses are all that are usually required to produce ordinary 

 telescopic objectives and eye-pieces. 



Perfect achromatism cannot be secured by these two older 

 types of glass, which are not exactly matched in dispersion 

 so as to give complete compensation for colour, and there- 

 fore leave minute outstanding colour fringes or uncorrected 

 " secondary spectrum." The improvement of optical glass 

 from this point of view was first systematically investigated 

 by Harcourt in 1834, this experimenter being, later, joined 

 by Stokes. They discovered the effect of boric and phosphoric 

 acids in reducing dispersion, but the prisms of their glasses 

 were not sufficiently homogeneous to allow accurate measure- 

 ment of dispersions, though they proved the possibility of 

 reducing secondary spectrum. A trial melting of one of 

 their glasses was made by Messrs. Chance Brothers with 

 somewhat disappointing results. The researches of Schott 

 and Abbe achieved more in this direction and resulted in the 

 production of " telescope " flint, which effected a decided 

 reduction of the secondary spectrum. A glass of similar 

 properties has recently been produced by Messrs. Chance 

 Brothers. 



In the case of the telescope, it is necessary that the 

 object-glass should produce an image which, over a small 

 area, should be as free as possible from aberrations; but, 

 in the case of other lens systems, particularly photographic 

 lenses, the image has to cover an area which subtends a 

 considerable angle to the lens, and with any great departure 

 from the axis of the lens system defects in the image appear, 

 which are not met with in the case of the telescope. Further, 

 the image must be as flat as possible, so that it may be as 

 nearly as possible coincident with the photographic plate. 



The production of a sharp image, flat over a considerable 

 area, necessitates the use of glasses differing considerably in 

 their optical constants from the simple crown and flint 

 types. In particular, it is desirable to have two glasses 

 differing considerably in the value for v, but not very widely 

 in mean index of refraction. 



Abbe was the first to realise the types of glass desired to 

 achieve the requisite corrections of the image in the more 

 complex optical systems, and his inspiration was of the 

 greatest possible assistance to the early German experi- 



