72 
The President's Address. 
I. The construction of perfect objectives of very liigli 
magnifying power has long been an object of ambition 
amongst the most eminent opticians ; but the history of the 
progressive development of the achromatic objective is so 
well known to most of m}^ hearers^ that it would be super- 
fluous in me to expatiate upon the subject. It may, however, 
be remarked that the employment of crystallized substances, 
possessing larger indices of refraction than glass, was long 
since proposed by Sir D. Brewster and Dr. Goring, and 
attempted to be carried out by Mr. Pritchard ; but, unfortu- 
nately, the crystals capable of being thus employed possess 
the property of double refraction, and the disturbance of the 
rays thus produced defies correction. The diamond is, I 
believe, the only highly refracting transparent and colourless 
natural substance that exhibits no double refraction, but the 
expense of constructing diamond lenses would be enormous. 
If quartz could be rendered not only amorphous, but homo- 
geneous, by fusion in masses of sufficient size for the con- 
struction of small lenses, it would doubtless be a great 
acquisition to the optician, but this, it is feared, is unattain- 
able. Glass of high refractive power has, I believe, been 
advantageously employed ; it is obvious that, w^here the index 
of refraction is high, less curvature will suffice for chromatic 
correction, and, consequently, there w^ill be less spherical 
aberration to contend with. 
Objectives of -rrth and of an inch focus have been most 
satisfactorily constructed for many years. In the beginning 
of 1862 Messrs. Powell and Leal and accomplished the con- 
struction of a uVth of an inch objective, of extremely good 
defining power ; and, more recently, Mr. Wenham has, I am 
informed, succeeded in constructing a -Vth of an inch objec- 
tive. It must, however, be borne in mind that, in the con- 
struction of these very deep objectives, the difficulty of main- 
taining the correct curvature of such very minute lenses is 
immensely increased ; and, moreover, that each surface, be 
it ever so carefully polished, presents but a rocky shore for the 
ethereal waves of light to break upon, and each individual 
ridge or furrow (supposing them in all cases to be the smallest 
possible) will have a greater disturbing effect on the entire 
pencil, in proportion to the smallness of the surface of the 
lens through which it is transmitted. It hence appears to 
me that there must be some limit to the magnifying power 
of an objective, beyond which defining power must neces- 
plan, however, was tried many years ago, by the late Mr. A. Ross and my- 
self, but the results were by no means promising, and I have not thouglit it 
worth while to repeat any attempts in this direction. 
