PRESIDENTIAL ADDRESS, O 



aperture to the object better than formerly, and I am inclined to 

 think that the one-tenth inch will be the highest required. 



It is too early, however, to give any decided opinion on this 

 subject, as often these matters do not come up to the general 

 expectation. Besides this, the only lenses at present in the market 

 are those of Messrs. Zeiss and Co. — at least, so I believe — and 

 they are dear and not easily obtainable. I got a dry i — 6th inch 

 from them just before leaving England, and during the short time 

 I was able to use it, it seemed to be very satisfactory. 



One result of this invention is that probably a very fair number 

 of good second-hand lenses will come into the market and be sold 

 cheaply. Those, however, who are thinking of buying a high- 

 power lens, but are not in immediate need of it, would act wisely 

 in waiting a short time to see if lenses made of this glass can be 

 more easily procured. Messrs. Zeiss and Co. have produced a 

 series of lenses under Professor Abbe's calculations of this new 

 glass and a series of eye-pieces to go with them. Both objectives 

 and eye-pieces are corrected respectively for the foreign six-inch 

 tube and for the ten-inch English tube. The objectives are all of 

 wide angle, and are rather expensive, especially if compared with 

 Zeiss's other list of lenses, though not so if compared with 

 English and American first-class prices. The oculars known as 

 " compensating eye-pieces " are all numbered on a new system, 

 recommended by Professor Abbe, which give their magnifying 

 effect on the simple magnification of the lens. For example, an 

 inch lens magnifies alone ten times ; if the eye-piece marked 8 is 

 used with this, the total magnification will be 80 diameters. 



I have been in the habit of using this plan myself, as it is so 

 simple and convenient, and can strongly recommend it to others. 

 It would be a great advantage if all opticians adopted it, and also 

 if they made their eye-pieces so as to give whole instead of frac- 

 tional multipliers. Mr. Zeiss's new eye-pieces are numbered 1, 2, 

 4, 8, 12, 18, and I think there is one of 27. They are so made 

 that the length of tube is not altered in changing them, which is a 

 great improvement, as objectives are corrected for a certain length 

 of tube. They also can be used for ordinary medium-angled 

 lenses, but are not recommended for extreme angles. Now that 

 so much attention has been drawn to eye-pieces, it is to be hoped 



