956 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



fixed. The stage is attached to the limb. The focusing is by means 

 of rack and pinion only, without fine adjustment. The general design 

 and construction are so simple that the instrument is issued at a very 

 low price indeed. 



Wray's Microscope Screen.* — L. Wray, jun., thinks that all who 

 have ever used the Microscope must be painfully aware of the fatigue 

 and distress which prolonged work with it causes to the eyes, and 

 therefore describes a device which he has been trying with this object 

 in view. When the eyes are exposed to a bright light, and one of 

 them is then covered over, the pupil of the uncovered eye at once 

 enlarges, and he believes this action of the iris to be the cause of the 

 distress produced in the use of the Microscope, for the pupil of the 

 working eye is unduly enlarged by the other eye being either shut or 

 shaded by a black screen, consequently more light is admitted to the 

 retina than it can comfortably bear, and the irises of both eyes are in 

 a state of tension, the one tending to contract and the other to 

 expand. 



The way in which he counteracts this is by exposing both eyes to 

 an equal light, by attaching to the eye-piece a cardboard screen, 

 which has two holes cut in it, the one to fit on the eye-piece, and 

 the other to allovs^ a thin piece of even-grained white paper being 

 presented before the eye that is not in use. 



A back view of the screen is shown in fig. 148, with the paper 

 removed. The two lines A A are intended to represent elastic 



Fig. 148. 



bands, by which the squares of paper are kept in place, as indicated 

 by the dotted lines, one, two, or more thicknesses being used, accord- 

 ing to the brightness of the field and translucency of the paper. The 

 object being to illuminate both eyes equally, it will be found con- 

 venient to gum on one thickness of this paper, and to have two or 

 three loose slips to adjust the amount of light. 



The plan is one that any one can try for himself; but a more 

 refined method of accomplishing the same thing is to have a ground 



Engl. Mech., xl. (1884) p. 180. 



