THE USE OF THE BULL'S-EYE 



409 



FIG. 845. Altered relations between lamp 

 flame and bull's-eye. 



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 -D 

 on the focal length of B. The 

 shorter the focus, the more in- 

 tense will be the light. 



We are here assuming 

 throughout that the field lens is 

 at a fixed distance from the 

 bull's-eye B. 



But if we move the flame, E 

 still central within the focus 

 of B, we get the result shown 

 in D, fig. 345. But by moving 

 E flthout 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 pi*incipal 

 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 in 

 illumination, it must 

 be placed so that E is 

 not at its principal focus, but at its centre of curvature. 



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 flame, 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 

 focussed on and centred to an object. Let 



FIG. 846. Result of placing flame in principal focus 

 of concave mirror. 



FIG. 347. Mode of obtaining critical image. 



when 

 this be 



condenser is 

 understood as 



