572 M. A. AINSLIE OX AN ADDITION TO THE OBJECTIVE. 



reduction of the average refractive index in the space between 

 the front lens and the object, except that in the case of the sub- 

 stitution of water for oil the effect is greatly increased. 



It was quite obvious, the moment I tried the experiment, that 

 mere increase of tube-length would not by itself make an oil- 

 immersion l/12th work as a water-immersion, on any cover- 

 glass that was likely to be met with ; but as soon as the idea 

 of the additional lens, as described above, occurred to me, it 

 immediately suggested itself as a way out of the difficulty. But. 

 as a rule, in the case of the conversion of an oil-immersion into 

 a water-immersion, the power of the additional lens has to be 

 much greater than is required with a dry lens, to correct for an 

 unusually thin cover-glass. In fact, it is generally necessary 

 to use a lens of such great power that the pencil of rays emerging 

 from the back lens of the objective is actually divergent instead 

 of convergent ; so that the correct position for the image has 

 actually " passed infinity " and the tube-length is negative, or, 

 in other words, the best image that can be formed by the ob- 

 jective is a "virtual image" several inches below the object! 

 It is hardly necessary to say that this means upsetting all the 

 corrections of the objective, and at first sight it does not look- 

 very promising. 



But on trial it was found that it was only the extreme margin 

 of the objective that was adversely affected. With an illuminat- 

 ing cone of not more than about 0'75 or 0'8 N.A., the definition 

 becomes quite satisfactory, and it appears that the central 

 portion of the objective is not to any great extent affected by 

 the violence done to it. 



Here again, as with dry objectives, an objective of moderate 

 power is much more amenable to the action of the additional 

 lens than one of very high power. The Zeiss l/7th-in., of N.A. 

 about 0'93, only requires a convex lens of 2 diopters, or 20 in. 

 focus, to effect the conversion ; a Leitz l/10th-in., the focus of 

 which is about 2*75 mm. (so that it is really a l/9th-in.), requires 

 8 diopters, or 5 in. focus ; and a Watson " Parachromatic ' 

 l/12th-in. (actually a l/14th-in. of N.A. 1*30) requires a lens of 

 10 diopters, or 4 in. focus. I have not experimented with any 

 stronger lenses, nor do I think that this arrangement would be 

 of much use with such. 



