718 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



We next have to centre the bull's-eye to the Triceratium by 

 moving the mirror, fig. 169. It will be noticed that centering the 

 bull's-eye does not put the light right. This must be done by 

 moving the lamp with its attached hulV s-eye. This movement must be 

 a kind of rotation of the lamp in azimuth round the wick as an ideal 

 axis. The relative positions of the lamp and bull's-eye must on no 

 account be altered. It is taken for granted that the bull's-eye is 

 fixed to the lamp, and was adjusted at the first so that the picture, 

 fig. 165, was obtained by direct inspection without any Microscope. 



Fig. 165. 



Fig. 166. 



Fig. 167, 



Fig. 168. 



Fig. 169. 



Fig. 170. 



This adjustment being satisfactorily carried out at first, is not dis- 

 turbed. By " moving the lamp round the wick as an axis," is meant 

 the moving of the whole thing as a solid mass. This is a very 

 simple thing to demonstrate practically ; but it is not easy to describe 

 even such a simple movement so as to preclude the possibility of 

 error. A very slight movement in the right direction will produce 

 the picture fig. 170. 



Any one having the necessary apparatus, by following out precisely 

 this plan, will arrive with very little trouble at fig. 170. 



All that need now be done is to open the full aperture of the 

 condenser, and put in the smallest opaque central stop ; if this does 

 not stop out all the light in the bull's-eye, then a larger one must be 

 tried. It is of the greatest importance that the stop be as small as 

 possible ; a very little difference in the size of the stop makes a great 

 difierence in the quality of the picture. Condensers ought, therefore, 

 to be supplied with as many opaque central stops as open apertures. 



