252 T. F. SMITH ON ARACIINOIDISCUS. 



much for colour, enough spherical aberration was left to seriously 

 impair the definition for large apertures ; on the other hand, to 

 correct perfectly for spherical aberration was to leave some out- 

 standing colour.- 



This was shown several years ago by Dr. Woodward, who proved 

 that when the " beads " of Formosum were projected on a screen 

 through a white corrected glass, they could not be separated. 

 When, however, projected on a screen through a glass which 

 showed the markings as ruby on a green ground, they were per- 

 fectly defined. There the matter rested up to the last two years, 

 and all the best glasses of every maker were under-corrected for 

 colour, but perfectly corrected for spherical aberration. What the 

 new glass has done, and all that it has done, is to enable an 

 optician to produce an objective that shall be without colour, except 

 what belongs to the object, and without sacrifice of definition, and 

 I can bear my testimony to the beauty of the image when defined 

 under these circumstances. 



Here, then, is the ideally perfect ; but it is one thing to make 

 such an object-glass, and quite another to bring it within the reach 

 of all willing purchasers ; and I am afraid that at present at least 

 the cost and the difficulty of producing them in sufficient numbers 

 will stand in the way of their general adoption. 



Side by side, however, with this advance to optical perfection 

 there has been progress of another kind, which, although con- 

 ducted in silence, seems to me of equal importance as bringing it 

 within the reach of the many w T ho, although desiring perfection in 

 optical appliances, have but limited means to acquire it. An 

 objective can be made wholly or partially of the new glass, and, in 

 my experience, the improvement in definition will be equal in each, 

 the only difference being that the one will give a coloured and the 

 other a colourless image. 



It is the importance of this other work I wish to impress upon 

 the members of this Club, and if in connection with it I am 

 obliged to mention names, it is because I know but of one English 

 firm which is doing this work, and feel that unless the new glass is 

 embodied in all new objectives there is no chance of improvement 

 in those produced at popular prices. 



The \" I am showing to-night is made by Messrs. Swift and 

 Son, and they tell me that by using the new optical glass in part 

 construction of their lenses, they have been enabled to enlarge the 



