212 president's address. 



N'evertheless, in tlie construction of highly-corrected lenses of 

 high power, without fluorite, there has been and is a distinct 

 advance, and this is the more important because it means the 

 production of constantly lower-priced lenses with high quality — 

 the very condition needed to promote the progress of micro- 

 scopy. 



And in this relation it would be an impropriety not to 

 remember the valuable contribution to the improved theory and 

 practice of lens-making provided by Professor Silvanus Thomp- 

 son in his paper to the Society of Arts, on " The Measurement 

 of Lenses," carrying with it as it does a most important " New 

 Focometric Method " and a beautiful newly-devised Focometer. 



It is not new, of course, for the optician to make exact 

 measures of optical quantities. Optics involves, as a matter of 

 course, exact methods ; but as a rule they are both costly and 

 complicated, and to have relatively easy means of testing with 

 severe accuracy every part of the microscope associated with its 

 optical functions will be to accomplish that most desirable of all 

 things in the interests of many sciences, i.e., make thoroughly 

 accurate and at the same time low-priced microscopes — as well 

 as cameras, telescopes, and other optical instruments — accessible 

 to students. 



What is really needed is a uniform system of describing the 

 properties of a lens. For all that is really placed at the dis- 

 posal of the student through the accessible sources of informa- 

 tion, the whole subject might be supposed to be exhausted by 

 considering the particular case of thin lenses. Prof. Thompson 

 shows how all the properties of a lens could be indicated by 

 specifying the position of four points, the two focal points and 

 the two " Gauss points," where the principal planes of the lens 

 intersect the action of it ; and by the apparatus he has devised 

 these latter points can be determined in any lens or combination 

 of lenses. 



There can be but little question that there is need in the in- 

 terest of English science for more accurate methods and broader 

 and deeper — as well as special — knowledge on optical matters. 



The establishment of an optical laboratory at Kew and else- 

 where in this country should be fostered by all who are 

 interested in the production of the highest class optical power 

 in all directions of scientific research. It is time for this 



