658 PROCEEDINGS OF THE 



convex and concave, of about 3 diopters power, or about 13 in. 

 focus, will suffice to correct for a very considerable range of 

 cover- thickness ; but with higher powers, such as a l/8th, the 

 amount of correction which can be got in this way is a good 

 deal less. This might be expected, from their greater sensitive- 

 ness to cover-thickness. As an example of what can be done 

 with an objective not too high in power, it may be said that a 

 Watson l/6th, of N.A. 0'74, which is normally corrected for a 

 cover 0'18 mm. thick, and a tube-length of 200 mm., can be 

 made to work well through a cover-glass as much as 0"50 mm. in 

 thickness, if a concave lens of 8 diopters be placed behind it, 

 while with a convex lens of the same or somewhat lower power 

 it will work well on an uncovered object ; and most other objec- 

 tives of this power will do as well. I have so far only experi- 

 mented with simple lenses ; but the chromatic and spherical 

 corrections of the objective are not perceptibly affected, unless 

 the power of the additional lens is as much as 10 diopters, and 

 even then the effect is not serious, and is not appreciable at the 

 centre of the field. The magnifying power of the objective is 

 somewhat reduced by the convex lens, as well as the N.A., 

 while with the concave lens the effect is the opposite ; but the 

 change is not great if the additional lens is placed as near to 

 the back lens of the objective as possible, though it does very 

 well in practice to place it behind the objective mount. There 

 is yet another use to which this additional lens may be put, 

 which, so far as I know, has not been previously described. If 

 for the oil in which an oil-immersion objective is immersed we 

 substitute water, the effect is just the same in kind as that of a 

 reduction in cover-thickness, though greater in degree ; and it 

 has been found possible to convert an oil-immersion into a 

 very good water-immersion by merely fitting behind it a convex 

 lens of suitable power. The power of the convex lens cannot 

 be predicted, but must be determined by trial for each objective. 

 It is easier to effect the conversion in the case of an oil-immer- 

 sion of moderate power, such as l/10th, than in the case of a 

 1/1 2th or higher power, though a 1/1 2th can be dealt with very 

 satisfactorily if its working distance is not too small. A Watson 

 tl Parachromatic ' l/12th, for example, requires a convex lens 

 of 10 diopters. It is important, in the case of a lens of this 

 power, to place the additional lens as near as possible to the 



