A Neio Semi-apochromatic ^. By E. M. Nelson. 657 



kind of image this lens gives of bacteria) were next examined, and 

 the flagella of the tubercle bacillus could be seen without any special 

 difficulty. 



Now with regard to its working distance, this was not directly 

 measured, but when the slide upon the stage was turned upside 

 down, it was found that the lens would just focus the object 

 through the slip. The thickness of the slip was then measured by 

 a screw micrometer as being ' 064 in. ; this divided by the refrac- 

 tive index, say 1*52, is • 042, or gV i^^- "^^^^^ lens is normally 

 corrected for an 8-in. tube, which is very handy, as it suits either 

 a long or short body. 



One more point before dismissing this subject. The measured 

 sensibility for the tube-length correction of this lens is just about 

 half that of modern semi-apochromatic sixths — a property which 

 will, of course, prove very acceptable to a biologist, as his mind 

 will be to a great extent relieved from anxiety and strain concerning 

 a correction about which his ideas are in general hazy and undefined. 

 But while it is unimportant to a microscopist, who can in a few 

 seconds deftly bring his objective into best adjustment, it will 

 nevertheless be the subject of considerable interest to him, because 

 first-rate objectives seldom possess this useful property of insensi- 

 bility to tube-length adjustment. Some few examples of fine old 

 objectives held this quality in a marked degree, but most modern 

 semi-apochromats require the tube-length to be closely watched 

 if fine results are to be obtained. 



The manifest usefulness of a lens of this description is my 

 excuse for bringing it to the notice of the Society. 



if this object will become sharp by re-adjustment of focus, then the fault is curva- 

 ture of image ; but if it will not become sharp upon focal re-adjustment, the error 

 arises from focal lines. Curvature of image is quite an unimportant error in a 

 Microscope objective, because all critical observations should be made in the central 

 portion of the field, the rest of the field being used merely as a finder. If it is 

 necessary to view large masses of an object, a lower power should be used. Sharp 

 central definition is not always compatible with flatness of field, and this sharp 

 central definition should never be sacrificed for what, at best, is only of small 

 importance. 



