DR. W. ROSENHAIN 12T 



There is one other direction in which I think that higher resolving 

 power is at any rate conceivable. Resolving power is a function 

 of the nuiQerical aperture expressed in terms of u sin a. Sin a 

 cannot be increased very much, but what about u ? The immersion 

 liquid is a difficulty, but I think that a higher refractive index for 

 the front glass is at any rate a thing where there is hope of success 

 as the result of research. I agree with Sir Herbert Jackson that 

 research will make it possible to make glasses of almost any desired 

 kind; there is, however, a good deal of emphasis to be placed on 

 the " almost," because the range of possible glasses is strictly con- 

 fined within certain limits of refractive index. I have on a previous 

 occasion given the values of these limits, and the limitation is due 

 apparently to quite definite physical and chemical causes. Glasses 

 having very low refractive indices or high ones, and having abnormal 

 optical properties, are virulent chemical agents in their action on 

 everything they may come in contact with, including air. They are 

 rapidly attacked by moist air, and they crystallise during manufac- 

 ture. There, I think, lies the solution of the problem. When we 

 look for substances which have high refractive indices, we find them 

 in crystals, and I want to carry that suggestion one step further. I 

 made it many years ago, but with the renewed stimulus to research 

 in this direction, it is worth making it again. The time has surely 

 come when we should meet this question of crystalline substances 

 for optical purposes by attempting to grow crystals artificially. I am 

 quite- sure that it can be done, and it ought to be done. I have 

 made a few preliminary experiments of that kind, and have succeeded 

 in producing some small calcium carbonate crystals. They were 

 small, but they were large enough for short- focus lenses, and I think 

 the idea of growing crystals is not altogether out of the range of 

 practical possibilities to-day. 



