Refraction by non-homogeneous Media. 713 



diffuse into the gelatine, increasing its refractive index. 

 The condition at the end of a few minutes will be not unlike 

 that of a white body surrounded by a dense atmosphere, for 

 the refractive index will be high at the boundary between 

 the jelly and glycerine, gradually decreasing as we pass out 

 into the jelly. The magnification resulting can be seen by 

 looking through the side of the trough, the lower portion of 

 the cavity appearing swollen out like a mushroom. If we 

 perform the experiment with pure glycerine and clean water 

 the same thing happens. By placing an arc light behind 

 the tank and throwing an image of the cavity upon a piece 

 of ground glass with a camera objective, placed at the centre 

 of the shadow of the tank, we can see the bright ring of 

 light which appears to surround the bottom of the cavity. 

 This is analogous to the ring of light which would be seen 

 surrounding the earth by an observer on the moon during a 

 lunar eclipse, or rather a solar eclipse. As the glycerine 

 penetrates into the jelly this ring of light eventually separates 

 from the line of the cavity. Photographs of this experiment 

 are reproduced in fig. 10, Plate XIV. 



Exner has described experiments with pseudo-lenses made 

 by immersing gelatine cylinders in water, and drying 

 sections of gelatinous cylinders. These I described in 

 4 Physical Optics,' but have since improved the method by 

 the use of glycerine. The whole experiment can now be 

 performed within the limits of the lecture hour. 



A handful of photographic gelatine is soaked in clean 

 water until thoroughly softened. The excess of water is 

 poured off and the mass is then heated until quite fluid, and 

 filtered through a funnel with a small piece of absorbent 

 cotton placed at the bottom of the cone. If the gelatine 

 refuses to run through, add a little more boiling water. 

 Pour a small quantity into a test-tube, and let it stand until 

 solid. Evaporate the remainder over a small flame, stirring 

 constantly until it is of the consistency of syrup. This 

 means boiling it down to one-third or less of its original 

 volume. Now add an equal volume of glycerine, and pour 

 the mixture into a second test-tube. After the jellies have 

 set, crack the bottom of the tubes by a sharp blow, warm 

 them by the momentary application of a Bunsen flame and 

 push out the cylinders. 



Cut the cylinders into disks of different thicknesses, with 

 a warm pen-knife. The best thickness is about two-thirds 

 of the diameter. Mount the disks between small squares of 

 thin plate-glass (window glass will do), warming the plates 

 slightly, to insure getting the jelly into optical contact. 



