M. A. AINSLIE ON AN ADDITION TO THE OBJECTIVE. 573 



Even if we have to sacrifice a little of the aperture of the lens, 

 however, it seems to me that there is a distinct field of utility 

 for this method of conversion. The effect on the definition, at 

 any rate in the centre of the field, of the additional lens is ex- 

 tremely small ; and with a l/12th-in. oil-immersion, treated in 

 this way, I have been able to get clear and strong resolution 

 of Amphipleura Lindheimeri in styrax, which, though not an 

 exhaustive test for a water-immersion, yet requires a pretty 

 good lens to give a really good image. 



The useful limit of N.A. for a lens treated in this way is, I 

 think, about 1*15. It is not very likely that the full aperture of 

 the objective would be available, when one considers that the 

 full N.A. of an oil-immersion treated in this way is something 

 like 1*29 out of a possible 1'33, and that no water-immersion has 

 so far been put on the market, even apo chromatic, with a greater 

 N.A. than 1*25. If we are content, however, to sacrifice a little 

 of the margin of the lens, we can get a good water-immersion of 

 about the aperture named, which should be useful for occasional 

 use at any rate, when it is not worth while going to the expense 

 of a proper water-immersion objective. 



It is a somewhat unfortunate, though unavoidable, circumstance 

 that the introduction of the additional lens shortens the working 

 distance, considering the limited working distance already 

 possessed by the average oil-immersion ; at the same time, the 

 l/12th-in. alluded to above will work through a cover-glass as 

 much as 0'20 mm. thick, and it is easy to obtain cover-glasses 

 thinner than this ; and it appears to me that it is only on tem- 

 porarily mounted specimens, such as films of living bacteria, 

 and the like, that one would want to use a water-immersion, the 

 great superiority of an oil-immersion on any permanently mounted 

 object being undeniable. 



For biological, medical, and other work that requires the 

 examination of living objects, I think there is a real sphere of 

 usefulness for this method of conversion. But at the same 

 time, it should be noticed that the advantages of the method 

 are more pronounced if the oil-immersion objectives employed 

 are of medium power, and a l/10th-in. is certainly more 

 suited to the additional lens than a l/12th-in. 



The conversion of an oil- into a water-immersion is particularly 

 useful when it is desired to examine living bacteria on a dark 



