50 Modern Microscopy 



used to cut off the marginal portions of a beam of light, 

 and in this sense is referred to in this book. The diaphragm 

 is often improperly called a stop. 



Diffraction Spectra. — If we look through a finely-ruled 

 grating at a gas or candle flame, we shall see a large number 

 of images of that flame having the colours of the spectrum. 

 This effect is due to diffraction. In the microscope, objects 

 having fine and regularly spaced markings diffract the light 

 in a similar manner, the resulting diffraction spectra being 

 plainly visible at the back of the objective. According to 

 the Abbe theory of microscopic vision these diffraction 

 spectra determine the character of the image seen, the 

 latter becoming less like the real structure when the number 

 of diffraction spectra admitted by the objective is reduced, 

 a faithful representation of the object being obtainable only 

 when all diffracted light of sensible brightness is admitted. 

 A further note on this interesting subject by Dr. G. John- 

 stone Stoney, F.E.S., will be found in an appendix on 

 page 124. 



Female and Male Screws. — The former is a threaded 

 fitting which receives a screw, and the latter a screw which 

 goes into the female fitting. In the case of a bolt and nut 

 the former would have the male, and the latter the female 

 screws. 



N.A. — Abbreviation for numerical aperture. See 

 page 60. 



Negative Eyepiece. — This is an eyepiece for examining an 

 image formed at the diaphragm set between the two com- 

 ponent plano-convex lenses. The Huyghenian is the best- 

 known form of negative eyepiece. 



O.I. — Abbreviation for optical index. See page 64. 



Positive Eyepiece. — This is an eyepiece for examining an 

 image situated beyond the field lens. It can consequently 

 be used as a magnifying-glass, etc. 



Refractive Index. — When a ray of light passes obliquely 

 from one medium into another of different density, the path 

 of that ray is bent or altered in its course. According to 



