LIGHT REFLECTED TO A FOCUS FROM THE OPEN SKY 413 



FIG. 354. With the open sky, light is 

 focussed at all points. 



illumination of which the object is its apex, no matter what the in- 

 clination or distance of the mirror. The angle of the cone will be 

 the angle the mirror subtends at the object subject of course to its 

 not being cut down by a stop. 



As a matter of fact, the boundless sky is an abstraction which is 

 never obtained in practice ; therefore it practically does make a 

 difference whether the plane or concave mirror is used, and whether 

 the latter is focussed on the object or not. 



The dotted lines in fig. 354 show rays falling on six different 

 points on a plane mirror: the continuous lines show the reflexions 

 of these rays on the object. 

 The heavy lines from either 

 extremity of the mirror to the 

 object show the maximum angle 

 of cone that mirror will give 

 in that particular position. 



The influence of a limita- 

 tion (as by means of a window) 

 should therefore be considered. 

 The extent to which it is 

 limiting, so far as its influence 

 upon the illuminating cone is 

 concerned, is shown by an ex- 

 amination of the back of tin- 

 lens of the objective when 1 1 it- 

 eye piece is removed. Fig. 3.55 shows the back of the objective when 

 the plane mirror is used, and fig. 349 R. when the concave mirror 

 is used, as in fig. 351. The beginner should study these experiments 

 by repeating them. 



Fig. 35(5 illustrates the method of obtaining dark-ground illumi- 

 nation when the arrangement shown in fig. 347 or 348 does not give 

 a sufficiently illuminated area even when 

 the flat of the flame is used. Of coin ' 

 it will be understood that for the dark- 

 ground result a suitable stop is inserted 

 beneath the sub-stage condenser. 



It has been shown by many illustra- 

 tions on many subjects that certain results 

 in critical work can be obtained with the 

 bull's-eye which are not so accessible with- 

 out its' use. But Mr. T. F. Smith baa 

 made this clear regarding the structure of certain diatoms. 



This, there can be no doubt, is due to the fact that the parallel 

 rays, falling on the sub-stage condenser, shorten its focus and in- 

 crease the angle of the cone of illumination. It will be noticed that 

 when the bull's-eye is introduced the condenser will need racking - 

 up. At the same time we prefer illumination as in fig. 347 or ;>4S. 

 except in cases where illuminating cones of maximum angles are 

 required. Thus it will be little needed with transmitted light 

 except when oil -immersion objectives of large aperture are used, 

 because illuminating cones up to -9 X.A. can be obtained with good 



FIG. 355. Image at the back 

 of the objective when day- 

 light and a plane mirror are 

 used. 



