114 ARRANGEMENT OF APPARATUS, ETC. 



position of its reflected image in the mirror behind it 

 should be observed, and if not coincident with the 

 flame itself, it is made so by tilting the tin shade sur- 

 rounding the lamp. The bull's eye is again brought 

 into view and centered. If the sun-light is not yet 

 perfect or if it appears as at B, the lamp is moved 

 more to the left or the right according to the posi- 

 tion of the bright ray proceeding from the aperture 

 of the diaphragm. These manipulations are greatly 

 facilitated by remembering the relation the bright ray 

 bears to the position of the lamp and bull's eye ; for, 

 when considered as pointing from the periphery to the 

 central hole, it indicates directly the position of the 

 lamp and bull's eye with regard to the optic axis of 

 the instrument. At the same time the distance of 

 the bull's eye from the lamp may be slightly altered, 

 care being taken to keep it central. When all other 

 means have failed, a very slight alteration of the 

 position of the condenser by the centering screws, 

 will sometimes cause its almost magical appearance. 

 For further details we refer the reader to Dr. 

 Dallinger's communication. 



Having obtained the sunlight by some means or 

 another, we rack back the objective. Now if the 

 object is flat with fine markings on or near its sur- 

 face we place it on the stage, but should it consist of 

 delicate scarcely differentiated structure, lying on dif- 

 erent planes, we replace the pin hole cap, which, by 

 the by, should itself be accurately centred, and if not, 

 rejected, and another tried. With the objective cor- 

 rected/or uncovered, (the ultimate correction for thick- 



