9G 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



Fig. 6. 



To allay any fear concerning the heat radiated or conducted from 

 the lamp, it may be stated that the lamp was attached to various 

 objectives and ran continuously at 100 p.c. over-voltage for more than 

 half an hour without doing any harm to the objectives. Dr. E. M. 



Chamot, of Corrjell University, conducted an 

 independent, series of experiments in which he 

 drilled a hole in the side of the objective, in- 

 serting a small pyrometer tube between the 

 lenses. He burned the ;lamp continuously over 

 long periods and pronounced it harmless. 



The Contrast Sensibility of the Eye as a 

 Factor in the Resolving Power of the Micro- 

 scope. — F. E. "Wright. The following sum- 

 mary is reprinted from the Journal of the Optical 

 Society of America, I'^l^, 2-3, Nos. 3-6. May- 

 November. Attention is directed to three factors 

 which are of importance in high-power microscope 

 work, namely — («) the use of a polarizing prism 

 to eliminate that part of the field light which 

 does not contribute to the diffraction pattern in 

 the image, and hence tends to reduce the con- 

 trast and to decrease the sharpness and crispness 

 of the image. This pihenomenon arises because diffracted beams which 

 emerge from gratings whose interval is of the order of magnitude of 

 half-a-wave length of light are sensibly polarized in a plane normal to 

 the lines of the grating. (J)) A diaphragm of the rectangular type is 

 recommended for use in the image plane of the eye-piece in order to cut 

 out all light except that from the particular object under examination. 

 This device allows the eye to work at best efficiency because it is not 

 disturbed by extraneous light. For the same reason the writer uses a 

 soft rubber eye-shield fitted to the microscope to cut out extraneous 

 light from the sides. Rubber eye-shields of this kind have been 

 employed by the Army and Navy for many years on fire-control and 

 other observing instruments. A dark screen or curtain of velvet is also 

 sus]">ended Ijetween the observer and the strong source of light and 

 shields his eye from the intense rays which fatigue them quickly, (c) The 

 importance of a field intensity of illumination approaching that of day- 

 light and best adapted for the eye at any particular time is emphasized ; 

 the simplest method for securing this is by means of a substage polarizer 

 which can be rotated, and with it the intensity of illumination of the field 

 varied. 



These factors are not important for ordinary observations, because 

 the resolving power there required is not great ; but in high-power, 

 critical work they are significant and enable the observer to accomplish 

 with comparative ease that which under other conditions is a matter of 

 difficulty. J. E. B. 



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