2 7 6 



Progress in Science. 



[April, 



spectroscope by a collar of soft leather interposed between them, cc, small holes 

 about one-eighth inch diameter drilled opposite to each other in the brass 

 tubes for viewing a magnified image of the reading scale, b, reflected in the 

 prism face, d. ef, a lens of short focus, and thin silvered glass diagonal 



Fig. 2. 



mirror, cemented on a cork, p, in the stopped eye-end of the telescope 

 tube, for obtaining a magnified view of the glass reading scale at B. The 

 latter is marked in transparent lines on an opaque ground, as shown in Fig - . 

 and is illuminated by the same direct light as that of which the spectrum 

 is ::--:'■ -:. 



We are indebted to Mr. R. C. Johnson for an account of a curious physical 

 phenomenon witnessed at Ziza — a lunar dew-bow. Ziza is a ruined city 

 which is situated about 20 miles £. of the northern part of the Dead Sea on 

 the table-land of Moab, and is about 3000 feet above the sea-level. The 

 Moabite Expedition was camping there on the night of the 24th February, 1872, 

 close to a large reservoir. Mr. Johnson says, " It was full moon (at n a.m. of 

 the same day), and having seen ducks come down to the reservoir, we turned 

 out about 8 p.m. to lie in wait for them against the sloping banks of the 

 reservoir. An exceedingly heavy dew had fallen, and I noticed when walking 

 with my back towards the moon that I was preceded by a faint circular halo 

 (extent of circle about | of circumference) ; the origin of which at first was 

 rather puzzling. On attentively considering the position of my eye and the 

 halo with regard to the moon, I found that it was exactly at the angle required 

 for an inverted rain-bow, and that it must really be a dew-bow. It seemed 

 brighter than a lunar rain-bow, which I have once beheld. Dr. Tristram also 

 noticed it after his attention had been called to it. I may also state that a 

 similar thing was seen in sunlight when a very fine dew was thickly spread 

 upon some large webs made by caterpillars in the same country. This was 

 looked out for after having previously seen the lunar dew-bow. 



An instrument invented in Germany for testing colour-blindness consists of 

 a rotating apparatus, which moves a disc whose centre is a circle, one half 

 black and the other white. Outside of this is a ring half red and half green, 

 then another ring of violet and red, then the outside ring of violet and green. 

 When rapidh T rotated, the centre appears to be coloured grey, that is black 

 and white mixed. To a green-blind person, the middle ring will appear grey, 



