494 SUMMARY OF CURRENT RESEARCHES RELATING TO 



layers of such a medium might be bright green and thick layers blood- 

 red. The principle is that the medium should transmit two distinct 

 regions of the spectrum, the absorption coefficient for one being greater 

 than for the other. Mr. Wood's method is to boil a quantity of Canada 

 balsam in an evaporating dish until a drop placed on a cold surface 

 becomes quite hard. A dye is made of commercial " brilliant green " 

 but must not be added until the balsam has cooled almost to the point 

 of becoming thick, otherwise it will be decomposed and produce a very 

 muddy green. Enough brilliant green must be dissolved in the balsam 

 to make it appear deep red in layers 1 • 5 cm. thick. This layer will be 

 found to be blue. It is desirable to add some naphthol yellow in 

 quantity sufficient to change the tint of thin layers from blue to green. 

 A hollow prism is now made by fastening two pieces of thin plate glass 

 between two grooved strips of wood. The base of the prism should be 

 about 2 cm. thick if the strips are 4 cm. long. The plates are warmed 

 with a flame and the coloured balsam poured between them. After the 

 balsam has cooled it is a good plan to run a quantity of melted sealing- 

 wax upon the top of it, which strengthens tbe prism. An incandescent 

 lamp or gas flame viewed through the prism is seen divided into a green 

 and a red image, the former gradually fading away as the eye is moved 

 towards the base of the prism. If a larger amount of the colouring 

 matter be added to the balsam and the fluid be pressed out between 

 pieces of plate glass, screens can be made which transmit a very good 

 secondary yellow. Through these screens a sodium flame is absolutely 

 invisible, though a gas flame appears of a colour very closely resembling 

 the soda flame. The colour of the transmitted light depends also on the 

 original composition of the light. By a suitable adjustment of the dyes 

 a screen can be made which appears red by lamplight and green by 

 daylight, illustrating very well the peculiarity of the alexandrite 

 crystals. 



Stopping Down the Lens of the Human Eye.* — W. Andrews- 

 suggests that the optical properties of the human eye may be improved 

 by using a metal plate with a perforation one-fiftieth of an inch in 

 diameter. This acts like a stop in a compound lens and renders un- 

 necessary the use of spectacles. 



Gerald Molloy points out that a pair of spectacles on the above 

 principle was made and used by the late Lord Sherbrooke, who was an 

 albino, and had no pigment in his iris. These spectacles consisted of 

 two convex metal cups closely resembling in size and shape the bowl of 

 an ordinary tea-spoon. In the centre of each was a small pin-hole which 

 was the only aperture through which light could enter. 



Merlin, A. — On the Critical Employment of the Microscope for Ordinary Working 

 Purposes. 



[A very useful paper, full of practical and valuable hints.] 



Journ. Quek. Micr. Club, VIII. (1902) pp. 195-209. 



(6) Miscellaneous. 



Holder for Metallurgical Work.— This apparatus (Fig. 104), made 

 by W. Watson & Sons, consists of two rotating jaws attached to the end 



* Nature, lxvi. (1902) pp. 31 and 56. 



