94 NOTES ON THE FUTUKE OF THE MICROSCOPE : 



which corresponds to an angular aperature of 144° (compare Fig. 2). 

 To go further in this respect appears to be impossible. On the one 

 hand the rays would no longer issue from the point lens ; on the 

 other hand, the most oblique rays like 01. (Fig. 2) would strike the 

 first dioptric plane at almost grazing incidence, and the losses by 

 reflection would become very considerable. 



Immersion Objectives — Keeping the angular value of the aperture 

 constant, the immersion increases the magnitude of n (Fig. 3). 



Cedar wood oil (n = 1.52) is mostly m.ade use of; it is interposed 

 between the glass cover and the front lens. The numerical aperture 

 may be raised to 1.40. In the great majority of cases nothing will 

 be gained by exceeding this limit. Medical men, botanists, histolo- 

 gists and bacteriologists study their specimens when immersed in water, 

 glycerin, salt solutions and, more rarely, Canada balsam. Only 

 in this last-mentioned case they really utilise the total numerical 

 aperture of their objective. For instance, when the object is placed 

 in water, with a numerical aperture of 1.40, we have numerical 



Fig. I. 



Fig. 2. 



Fig. 3. 



aperture utilised = 1.33/1.52 of total aperture = 1.22. The conclu- 

 sion to be drawn is that, taking into consideration only the theo- 

 retical definition and the customary practice, one may say that the 

 microscopic definition has already reached its limit. We have not 

 referred to the variation of A ; that point will be discussed when we 

 pass to microphotography. 



Special Cases, {a) Diatoms. — For diatoms and in general for very 

 fine and refractory objects, w^e can make use of a medium and an 

 immersion liquid of very high index. For the silica test of diatoms, 

 e.f/., we can apply a solid medium of relatively high melting-point, 

 as silica will stand high temperatures. In this way Zeiss has arrived 

 at an objective of a numerical aperture of 1.60 in making use of a 

 dens© flint (for slip and cover) of monobromonaphthalene (immersion 

 fluid), and a solution of arsenic sulphide in bromine (as medium). 

 There again we seem to have reached the limit. 



(6) Metallogrnphy. — In metallography the immersion liquid 

 touches the object directly without interposition of any lamella. In 

 this case there is no theoretical reason against the application of 

 extreme numerical apertures ; unfortunately the illumination problem 

 becomes particularly difficult. 



