THE USE OF THE BULL'S-EYE 



409 



obtained by placing the eye in the rays and by looking directly at 

 the bull's-eye. 



The light is so intense that it'-is more pleasant to take the field 

 lens of a 2-inch eye-piece and place it in the path of the rays focus- 

 sing the image of the bull's-eye on a card. It should be noticed 

 with care that the diameter of the disc A depends upon the diameter 

 of the bull's-eye B ; but the in- 

 tensity of the light in A depends 

 011 the focal length of B. The 

 shorter the focus, the 



more in- 

 tense will be the light. 



We are here assuming 

 thi'oughout that the field lens is 

 at a fixed distance from the 

 Inill's-eye B. 



But if we move the flame, E 

 still central within the focus 

 of B, we get the result shown FK; 345 ._ Altered relations between lam 



ill D, fig. 345. But by moving flame and bull's-eye. 



E without the focus of B we get 



the picture H, while K is the picture when E is focussed but not 



centred. 



A common error, one repeatedly met with, is that of placing a 

 concave mirror, C (fig. 346), so that the flame, E, is in its principal 

 focus. The result of this is that parallel rays are sent to B. These 

 rays are brought to a focus at a distance from B about equal to twice 

 the radius of the cur- 

 vature of B and then 

 scattered, a totally 

 different result from 

 what is aimed at. If 

 the concave mirror, C, 



is to be of any use ill FlG . 34 6. Result of placing flame in principal focus 



illumination, it must of concave mirror. 



be placed so that E is 



not at its principal focus, but at its centre of cur rat/ in . 



The bull's-eye gives an illustration of what is of wider application. 

 The method of obtaining 

 a, critical image with a 

 condenser by means of 

 transmitted light is 

 shown in fig. 347. E is 

 the edge of the name, S 

 represents the sub- stage 

 condenser, and F the 

 object. F is thus the 

 focal conjugate of E, and 

 F and E are in the prin- 

 cipal axis of S ; that is 



to say, these are the relations which exist when a condenser is 

 focussed on and centred to an object. Let this be understood as 



FIG. 347. Mode of obtaining critical image. 



