224 Royal Society : — 



As this photograph is not reproduced, it may be stated that the 

 ring-images have an internal diameter of nearly f- of an inch. 



The accompanying solar profiles are copies of drawings made, on 

 the dates stated, by means of the new method, which were exhibited 

 by the authors at the Meeting. 



[Since reading the above paper, it has come to our knowledge 

 that Zollner had conceived the same idea unknown to us, but had 

 rejected it. Prof. Wenlock in America has tried a similar arrange- 

 ment, but without success. — J. N. L., Gr. M. S., January 17, 1873.] 



January 16. — T. Archer Hirst, Ph.D., Vice-President, in 

 the Chair. 



The following communication was read : — 



"A new Formula for a Microscope Object-glass." By F. H. 

 Wenham. 



A pencil of rays exceeding an angle of 40° from a luminous 

 point cannot be secured with less than three superposed lenses of 

 increasing focus and diameter, by the use of which combination rays 

 beyond this angle are transmitted, with successive refractions in their 

 course, towards the posterior conjugate focus : until quite recently, 

 each of these separate lenses has been partly achromatized by its own 

 concave lens of flint glass, the surfaces in contact with the crown 

 glass being of the same radius, united with Canada balsam ; the 

 front lens has been made a triple, the middle a double, and the 

 back again a triple achromatic. This combination therefore consists 

 of eight lenses, and the rays in their passage are subject to errors 

 arising from sixteen surfaces of glass. 



In the new form there are but ten surfaces, and only one concave 

 lens of dense flint is employed for correcting four convex lenses of 

 crown glass : as this might at first sight be considered inconsistent 

 with theory, a brief retrospect of the early improvements of the 

 microscope object-glass will help to define the conditions. The 

 knowledge of its construction has been entirely in the hands of 

 working opticians ; and the information published on the subject 

 being scanty, this has probably prevented the scientific analyst from 

 giving that aid which might have been expected. 



Previous to the year 1829 a few microscopic object-glasses were 

 made, composed of three superposed achromatic lenses • but this 

 combination appears to have been used merely with the inten- 

 tion of gaining an increase of power, in ignorance of any prin- 

 ciple, and without even a knowledge of the value of angular 

 aperture. 



At this time the late J. J. Lister tried* a number of experiments, 

 and discovered the law of the aplanatic focus, and proved that, by 

 separating lenses suitably corrected, there were one or two posi- 

 tions in which the spherical aberration was balanced. This was 

 explained in a paper read before the Eoyal Society in 1829. In 

 the year 1831 Mr. Eoss was employed to construct the first achro- 



