ACCESSORIES 57 



skill and mathematical ability, and is completely 

 beyond the powers of the average microscopist. 1 



Although with the increase in power it is correspond- 

 ingly difficult to combine all these corrections in one 

 objective, they are brought to a high pitch of excel- 

 lence in the present-day "achromatic" objectives, and 

 so remove the necessity for the use of the higher priced 

 and less durable apochromatic lenses. 



In selecting objectives the best "test" objects to 

 employ are: 



1. A thin (one cell layer), even r T " 2 " 1/r 

 "blood film, "stained with Jenner's 



or Romanowsky's stain. 



2. A thin cover -slip prepara- 

 tion of a young cultivation of 

 B. diphtheria (showing segmenta- 







for ", -J-" 

 T y oil 



for 



i" dry 



tion) stained with methylene- 

 blue. 



Accessories. Eye Shade (Fig. 49). This piece of 

 apparatus consists of a pear-shaped piece of blackened 

 metal or ebonite, hinged to a collar which rotates on 

 the upper part of the body tube of the microscope. 

 It can be used to shut out the image of surrounding 

 objects from the unoccupied eye, and when carrying 

 out prolonged observations will be found of real service. 



Nosepiece. Perhaps the most useful accessory is a 

 nosepiece to carry two of the objectives (Fig. 50), or, 

 better still, all three (Fig. 51). This nosepiece, pref- 

 erably constructed of aluminium, must be of the 

 covered-in type, consisting of a curved plate attached 

 to the lower end of the body tube a circular aperture 

 being cut to correspond to the lumen of that tube. To 



1 Its importance will be realised, however, when it is stated in the words of 

 the late Professor Abbe: "The numerical aperture of a lens determines all 

 its essential qualities; the brightness of the image increases with a given 

 magnification and other things being equal, as the square of the aperture; the 

 resolving and defining powers are directly related to it, the focal depth of 

 differentiation of depths varies inversely as the aperture, and so forth." 



