368 



E. M. NELSON ON A NEW LOW-POWER CONDENSER. 



stop than is necessary.* How often one sees a dimly lighted 

 object, with a halo of bright fog, on one side of the field, owing 

 to the use of an excentric stop larger than is necessary. 



To remedy this defect, Mr. Curties shows a simple centring 

 stop-holder made from my design. The stop consists of a disc 

 with a hole in it which fits on a pin B ; this I designed for my 

 Jubilee microscope, which was made by Powell and exhibited at 

 the Club in 1887. 



Why microscopists will have their stops cut out of the sheet, a 

 much more expensive plan than a disc fitting on a pin on a spider, 



ZI 



v 



Fig. 4. 



A, lever ; B. flat tube with the stop on pin ; C shows the flat tube placed 

 on the lever, with screw for fixing the appliance beneath the iris-box. 



I am unable to tell you. But to return, this pin is fixed to the 

 end of a flat tube B, which slides on a flat bar A ; this forms the 

 centring adjustment right and left. The centring adjustment 

 rectangular to this is in arc, by moving the arm C, which is 

 pivoted below the iris box. 



* If, for example, a centred stop of -4-inch diameter is requisite, and 

 supposing that the stop carrier is 1 inch out of centre, then a stop of 

 6 inch will be required to do the same work as the stop of *4 inch. Now 

 the area of a circle of "6 inch diameter is more than double that of a circle 

 4 inch diameter ; this shows the great loss of light an excentric stop-holder 

 causes. 



Journ. Quek-ett Microscopical Club, Scr. 2, Vol. XII. , No. 75, November 1914. 



