1 905-1906.] Address by the President. 327 



who, in conjunction with the firm of Zeiss of that city, took 

 up the matter. After many experiments the oil of cedar-wood 

 was found to possess all the requisite properties, and then was 

 constructed the excellent series of oil-immersion object-glasses 

 — called also, from the refractive index being the same 

 throughout, homogeneous immersions. With these object- 

 glasses two important advantages were gained : there was no 

 loss of light at the cover-glass by total reflection — all the rays 

 which emerged from the object which the aperture of the 

 object-glass was capable of taking up were taken up ; and 

 again, a collar adjustment for different thicknesses of the 

 cover-glass was rendered unnecessary. The diagram on Plate 

 XXVI., Fig. 2, will help to explain the gain obtained by the 

 oil-immersion over the dry object-glass. 



Let A be an object mounted in balsam with a cover-glass, 

 and J the front lens of an object-glass of which the right- 

 hand side may be taken to represent an oil-immersion and the 

 left a dry one. Let A B be the central rays, — thus it will be 

 seen that the side rays 1, 2, 3, 4, and 5 are all admitted by 

 the oil-immersions, while only rays 1, 2, and 3 are taken up 

 by the dry objective, the rays 4 and 5 being lost by total re- 

 flection. In using these immersion-lenses one thing must be 

 kept in mind, — the object must be either mounted in balsam 

 or in some medium of equal refractive index ; or if mounted 

 dry, it must be in close contact with the cover-glass, otherwise 

 there would be a film of air between the object and the cover- 

 glass, in which case the latter would act simply as a front 

 lens, and thus convert the whole into a dry system. Another 

 point to be observed is immediately after use to wipe the oil 

 clean off both the object-glass and the cover-glass. This should 

 never be omitted. 



Apochromatic Lenses. — The manufacture of achromatic 

 object-glasses had now reached the highest stage of per- 

 fection possible with the materials — crown- and flint-glass — 

 which were at the command of opticians. But there was 

 still a desire for something better. With the crown- and flint- 

 glass two rays of the spectrum, as we have seen, could be 

 united, leaving a secondary spectrum of the other rays un- 

 corrected ; and for the correction of spherical aberration, one 

 ray only between the violet and the red could be taken. 



