PROCEEDINGS OF SOCIETIES. 139 



definition. It would occupy too much time to go fairly into the ques- 

 tion of a good stand, but I am gratified to hear that Mr. Browning is 

 engaged, in conjunction with Mr. John Mayall, jun., in perfecting a 

 new microscope for students which will embody every practical im- 

 provement, consistent with simplicity and the use of high-power 

 glasses. 



But there is one point about a good working instrument of even 

 more importance than a perfect stand, and that is a first-rate objective. 

 In selecting a magnifying power for scientific work of any kind, it 

 must be our endeavour to secure one giving the very best defini- 

 tion and penetration. These are two of the most essential quali- 

 ties in every good objective ; as on the first depends the truth 

 of the optical image, and on the second the proper appreciation 

 of its histological characters or structure. The defining power of 

 an objective, as I dare say most of you know, chiefly depends on the 

 perfection of the corrections for spherical and chromatic aberration. 

 A fourth of 120° and an eighth of 150° angular aperture may be looked 

 upon as standard objectives. Greater angidar aperture in dry objectives 

 is not in my experience beneficial for medical microscopy. Increased 

 angle of aperture frequently means impaired definition ; the explana- 

 tion of this is, that the manufacture of glasses with the utmost angle 

 of aperture is attended with increased difficulties, and requires the 

 most skilled workmanship. It is, therefore, a somewhat rare thing to 

 meet with an objective of great angular aperture that approaches to 

 freedom from spherical aberration. The absolute correction of chromatic 

 aberration in an objective is of far less importance than the correction 

 of spherical aberration, and just so is it less important to the histolo- 

 gist to have a colourless image than one with perfectly sharp defini- 

 tion. By this test will the student be inclined to estimate the value 

 of any object-glass. Increased angular aperture enables us to bring 

 to our aid, when needed for the resolution of an object, a more oblique 

 pencil of light than we otherwise could ; and we should be prepared 

 to employ every kind of illumination in our work. Indeed, we should 

 not in any case pass judgment upon a structure until an exhaustive 

 series of trials has been made upon it by every method of illumina- 

 tion. The cover-glass, as Amici long ago pointed out, exerts con- 

 siderable influence on the perfection of the image. An object or 

 preparation without a cover-glass gives a sharper image than one 

 covered. A thick glass cover increases spherical aberration. In the 

 immersion objective, the film of water removes or lessens many of 

 the evils inherent to the dry objective. In the immersion system the 

 stratum of water becomes, as it were, an adjustable film between the 

 objective and the object, and greatly assists in the correction of 

 spherical and chromatic aberration. As water is a stronger refracting 

 medium than air, the reflexion of the rays of light is much diminished 

 at the upper surface of the cover-glass, and on its incidence on the 

 objective ; here, indeed, it is almost entirely neutralized, and hence a 

 greater number of rays do actually contribute to the formation of the 

 image. The thin film of water produces very nearly the same effect 

 as enlarged angle of aperture. It also collects the peripheral rays 



