574 M. A. AINSLIE ON AN ADDITION TO THE OBJECTIVE. 



ground. It sometimes happens, when an oil-immersion is used 

 for this purpose, that the cover- glass has an unpleasant knack 

 of sticking to the front lens of the objective ; with a water- 

 immersion this difficulty is absent. iVlso, there is the distinct 

 advantage that it is easy to remove the water if it is desired 

 to examine the object with a dry lens, whereas this is by no 

 means an easy matter in the case of oil. The performance of 

 the Zeiss l/7th-in., used as a water-immersion, upon living 

 objects on a dark ground is especially good, though the Leitz 

 l/10th-in. is not far behind. 



In the case of an oil-immersion, it is well to have the additional 

 lens fitted as close to the back lens of the objective as possible ; 

 there is no difficulty in doing this, as it is usually the practice of 

 makers to supply a ' funnel stop ' to which the optical part 

 of the objective can be screwed. If the stop is removed, and 

 the additional lens fitted in its place, so as to be close to the 

 back lens when the optical part is screwed on, the effect on the 

 magnifying power is not serious, and can be disregarded. In 

 the case of the Leitz 1/lOth-in., for example, the objective in 

 its normal state has a focal length of 2"75 mm., and is a l/9th-in. ; 

 with a lens of + 8 diopters in position immediately behind the 

 back lens, the focal length is 3 mm., and the objective becomes 

 a l/8th-in. 



For dark-ground work, of course, the aperture must be reduced 

 to something like 0*85, with the dark-ground illuminators of the 

 present day ; and we may either fit a separate " funnel stop " 

 with a small lens in it, giving this aperture, or use the funnel 

 stop as it stands, and fit the lens in rear of the objective mount. 

 This, of course, reduces the power rather more than the other 

 arrangement, but this is not serious, as sufficient power can be 

 obtained by the use of a deeper eyepiece. 



It should be noted that when the oil-immersion has been 

 converted in this way to a water-immersion, it becomes sensitive 

 to variations in the thickness of the cover-glass, though not to 

 the same extent as a dry objective ; the draw- tube will as a 

 rule be able to deal with this, but if more correction is required 

 it can be obtained by means of additional lenses, in the manner 

 described above for dry objectives. 



I have left to the last, principally because it is more inter- 

 esting than practically useful, what is, from the " brass and 



