33? OPTICAL PROJECTION 



in proportion to the thickness of the film, thicker and fewer 

 in proportion to its thinness. 



188. Thick Plates. Newton's experiment with a thick 

 concave glass mirror is pretty easily projected with a good 

 light, using a large mirror optically worked on both sides, of 

 2 to 4 feet focus. This is to be slightly dulled on the surface 

 by laying on a film of milk and water ; milk alone is much too 

 opaque. A radiant point must now be adjusted at the centre 

 of curvature, so that the incident rays are reflected back and 

 focussed in the same point. With the arc light the best plan 

 is to place a rather small aperture on the lantern-nozzle, and 

 converge through it all possible rays from the condensers ; 

 then upon a screen of white card adjusted over the aperture, 

 with a small hole in the centre for the rays to pass through, 

 beautiful interference-rings will be seen, which will be very 

 brilliant if all is adjusted properly. The same plan may be 

 adopted with the oxy-hydrogen light ; or the naked jet may 

 be brought out of the lantern, and adjusted with the incan- 

 descent face to the mirror, and the back to the screen. This 

 latter will thus be shielded from the direct light from the 

 lime, further stopped back by a small circular stop behind it 

 if necessary ; while the rings formed in the air in the plane 

 of the lime can be fairly focussed on the screen, if the light 

 is very brilliant, with a large lens. 



189. Fresnel's Prism. The interference bands from 

 Fresnel's inclined mirrors are practically impossible to project 

 upon a screen. Even with his bi-prism, they are rather 

 difficult with the 0. H. jet, but easier with the arc light. 

 The best arrangement is to use the optical front, with the 

 objective removed and replaced by the plain nozzle, carrying 

 an adjustable perpendicular slit. In the optical stage is placed 

 a cylindrical lens, about 5 niches focus, and 2 inches diameter, 

 mounted in a wooden frame. This is to condense a large 

 amount of light upon the slit, light and lens being so ad- 

 justed that the parallel beam is focussed upon the slit. From 



