314 B. M. DRAPER ON THE GREENHOUGH BINOCULAR. 



As, however, the front lenses of the twin objectives stand out 

 some distance on either side of what would be the optic axis of an 

 ordinary microscope, the stop has to be correspondingly broad 

 from side to side ; otherwise direct rays would enter the objectives 

 and would spoil the dark ground at the sides of the field. But it 

 is not necessary that the rectangular diameters of the stop should 

 be equally great ; on the contrary, if an ordinary circular stop be 

 used, some rays are needlessly obstructed. On trial, a double 

 or twin stop, corresponding with the twin objectives, gave much 

 better results. This stop consists of two small circular patches 

 placed side by side in the same plane, and touching each other r 

 so as to form a figure of eight. It is used behind the condenser 

 in the same way as an ordinary circular stop, and with almost 

 equal ease. It is only necessary to be careful that the two circular 

 jDatches shall be placed horizontally, i.e. so as to be opposite the 

 two front lenses of the twin objectives. This position can easily 

 be secured by arranging the stop in the carrier approximately and 

 then, whilst watching the object, shifting the whole condenser 

 round in its sleeve until the best effect is obtained. A standard 

 low-power condenser such as Swift's " Paragon," with its top lens 

 off, gives very satisfactory results. The twin and the ordinary 

 circular patterns of stop were compared experimentally by using a 

 condenser fitted with two stop carriers, one behind the other, so 

 that either stop could be used separately, or both together. The 

 twin stop used by itself gave a good dark ground. The circular 

 stop was purposely chosen too small to give a good dark ground ; 

 there was light at the sides of the field. Nevertheless when the 

 circular stop was turned in above the twin stop whilst the object 

 was under observation, there was a marked drop in the brightness 

 of the image. This loss of light was due almost entirely to the 

 circular stop, not to the clear white glass on which it was mounted, 

 since it was found that the interposition of such a. piece of glass,, 

 even when rather dirty, made very little difference to the light. 

 Evidently, therefore, the circular stop, though too small in one 

 direction, was too large in the other, and kept out some rays 

 which might safely have been admitted. Of course if the circular 

 stop had been large enough to darken the background when 

 used by itself, the loss of light would have been still more 

 noticeable. 



