ZOOLOGY AND BOTANY, MICROSCOPY. ETC. 487 



able from any other transverse section of the fringe system. Clearly 

 in this case, the geometrical image is merely that section in which the 

 geometrical condition of similarity to the object is satisfied. 



The best arrangement of the experiment is the following. Light 

 from an arc lamp A is focnsed by means of the lens B upon a narrow 

 slip C. Thence it passes through a direct-vision prism at D, and the 

 spectrum is focused by the lens E upon the narrow slit of a collimator 

 F G. The parallel l)eam of monochromatic light thus obtained falls 

 upon the mirror H of a microscope K J, upon whose stage, at I, the 

 grating is placed in such a position that its ruled lines are parallel to the 

 projection of the two slits C and F. Using a black-line grating of 400 

 lines to the inch, and having both slits narrowed down to a small fraction 

 of a millimetre so as to secure very homogeneous illumination, the field 

 of view was examined with §-in. objective and 1-in. eye-piece. The 

 interference fringes appeared in the field of the eye-piece with exquisitely 

 sharp definition throughout the whole range of the coarse-adjustment of 

 the Microscope, i.e. over a distance of 58 mm., beginning with the front 

 of the objective in contact w^ith the grating and with its focal plane 

 7 mm. below the ruled surface ; and the fringes could be traced through 

 a much greater range by withdrawing the eyepiece and moving it back 

 along the axis. As the Microscope is slowly focused upward, the bands 

 undergo curious changes in appearance, the lines showing sometimes 

 close togetlier and again further apart, but the definition is almost 

 equally sharp throughout the whole range of adjustment, so that any 

 section of the fringe system is as good an apparent image as any other 

 section. Similar but less perfect effects may be obtained by illuminating 

 the field by means of sodium light passing through a slit a couple of 

 millimetres wide at a distance of one or two metres. 



If the angle of the incidence of the light on the grating is changed 

 by moving the mirror, the whole fringe system shifts to one side or the 

 other except in the focal plane, where it remains stationary. This shows 

 (1) that the focal plane is the plane in which the interference fringes 

 formed by light of all incidences coincide ; (2) that, when a broad 

 source is used, the geometrical image is really a superposition of co- 

 incident interference patterns ; and (8) that the usual absence of a sharp 

 image outside the focal plane is due to the more or less uniform illumina- 

 tion resulting from the overlapping of fringe systems due to light coming 

 from various points in the source. When the grating is illuminated by 

 a parallel beam of white light by means of a collimator with very narrow 

 slit, or, less perfectly, by a distant gas flame turned edgewise, the effects 

 are similar except that outside the focal plane the fringes are coloured. 

 Hence (4) the focal plane is also the plane of achromatic interference, 

 i.e. the plane in which the fringes due to light of various wave-lengths 

 coincide. 



These experiments show very clearly why it is in general essential to 

 use a condenser to illuminate the field of a Microscope in order to obtain 

 a critical image, i.e. an image which conies sharply into and out of focus 

 and which is hence as free as possible from confusion with details of 

 structure lying above and below the focal plane. It is interesting to 

 observe how, as the illumination is made less homogeneous and more 



