4 PRESIDENTIAL ADDRESS. 



really show more detail, and are not so trying to the eyes as bad 

 ones often are — we might find more enthusiasts, and fewer of 

 those who buy a microscope, and then get tired of its use. The 

 members of the Postal Microscopical Society might do a great 

 deal of good by popularising the use of the microscope, sweeping 

 away the mystery which surrounds it, and save people from 

 wasting their money in the purchase of inferior instruments. 



AVe should probably thus increase the number of students and 

 acquire new members, by infusing fresh blood into the Society, 

 give it new life and a large addition of brain power. An import- 

 ant advance in practical optics has just been made which will 

 probably have a great effect on microscopes — namely, the manu- 

 facture of a specially dense glass by Messrs. Schott and Co. This 

 glass has already been utilised by the well-known opticians, 

 Messrs. Zeiss and Co., in the manufacture of a special series of 

 lenses known as the " apochromatic objectives," and experiments 

 are now being carried on in England with the same glass. The 

 great advantage claimed by its use is the abolition of the second- 

 ary spectrum which exists in lenses at present, and gives rise to 

 the colour corrections of different opticians, the best of which is 

 the well-known ruby correction, which I believe was introduced 

 by Messrs. Powell and Lealand. A tertiary spectrum remains, 

 but its colouring power is weak, so the images of objects are prac- 

 tically free from colour, and a nearer approach is made to achro- 

 matism. The images thus produced are clearer and sharper, and 

 show their natural colouration better. The objectives themselves 

 thus approach much nearer to their theoretical performances, and 

 in consequence will bear much stronger eye-pieces. 



This is, of course, a great improvement. We all know by 

 experience the great nuisance of perpetually changing the objec- 

 tive in an investigation, an inconvenience which is not satisfacto- 

 rily obviated by any form of nose-piece that I have ever seen. For 

 instance, suppose a botanical section is being examined wanting 

 powers of about 40, 1 00, and 200. Formerly a one-inch, a half- 

 inch, and a quarter-inch were required for this. Now, with this 

 new advance, one power of an inch and three eye-pieces will answer 

 the purpose. I do not mean to say that high powers will no 

 longer be required, but that now we will be able to suit the 



