M. A. AIXSLIE ON AN ADDITION TO THE OBJECTIVE. 571 



The additional lenses above described are easily fitted behind 

 the objective if they are mounted in small cells which push 

 into a ring made to screw to the nosepiece, and having a thread 

 in front to take the objective. Otherwise they may, as stated 

 above, be fitted immediately behind the back lens, and almost 

 in contact with it ; but this is hardly necessary, unless it is 

 important that the power of the objective should be affected as 

 little as possible. The lenses I have had made are just over 

 1-i mm. in diameter, and the outside diameter of their cells is 

 rather less than 15*5 mm., so that there is ample room for 

 them to push into the upper side of the ring suggested, leaving 

 sufficient thickness in the ring for the thread to fit the nose- 

 piece. 



They were beautifully made and fitted for me by Messrs. 

 H. F. Angus & Co., who supplied me with a series of 11 of these 

 lenses, varying in power from +10 to 10 diopters. With 

 this series almost anything can be done in the way of cover- 

 glass correction. 



I have tried both biconvex and biconcave, and plano-convex 

 and plano-concave lenses, the latter with the plane side both 

 upwards and downwards, without being able to see any difference 

 in the performance ; biconvex and biconcave lenses are easier 

 to obtain, and I should recommend them to any one thinking 

 of trying this device. 



So far we have been dealing with the use of an additional lens 

 with dry objectives ; but I now come to a use for this device 

 which has not, as far as I know, been suggested before. I refer 

 to the conversion of an oil-immersion objective into a water- 

 immersion. 



Some time ago I found that with certain oil-immersion objec- 

 tives it was possible to get good definition with glycerine as 

 the immersion fluid if the tube length was increased by 60 mm. 

 or so. Was it possible to use these objectives as water-immer- 

 sions ? 



The substitution, in the case of an oil-immersion objective, 

 of a medium of smaller refractive index for the oil has an effect 

 on the corrections of the objective precisely similar in kind to 

 the reduction in the thickness of the cover-glass in the case of 

 a dry objective ; in each case the effect is really due to the 



