342 



NA TURE 



[February 9, F893 



noticed it before. Of course when the apparatus is in full 

 working order there is little opportunity for doing so. 



The only explanation I could think of was: — (!) that (he light 

 of the lamp had produced some molecular change in the paint 

 coating the notice board ; (ii) that this had affected differently 

 the blue and the white paint ; (iii) that the same cause had set 

 up some differential electrical condition of the board and the 

 glass ; (iv) that a bombardment of particles of the blue paint 

 had taken place on to the glass to which they had adhered ; and 

 that (v) the particles ?o adhering, by dispersing the light, pro- 

 duced the effect of the pale illuminated ground while the spaces 

 occupied by the letters being relatively clean stood out dark. 



Rnval Gardens, Kew, W. T. Thiselton-Dyer. 



February i. 



Mr. W. B. Croft's paper on Breath-Figures in your issue of 

 December 22 reminded me of some curious impressions of 

 monumental brasses which are to be seen on the walls of 

 Canterbury Cathedral. When I saw these impressions a few 

 years ago, it occurred to me that they might have been produced 

 by mere contact, the brass plates having possibly been hung for 

 many years against the walls, in secluded corners, at a time when 

 the Reformers would not let them remain in their proper 

 matrices on the church floor. I had forgotten the particulars of 

 these figures, but Dr. Sheppard, of Canterbury, has kindly sent 

 me the following notes by favour of Canon Fremantle :— " A 

 number of impressions of brasses are in the basement (which is 

 open to the air) under Henry IV. 's chantry in the Cathedral. A 

 very good impression is on the western column of the crypt of 

 Trinity Chapel. ... On the walls appear the shapes of 

 the effigies. Sometimes the stone is unstained all over the area 

 of the figure, and surrounded by a broad dark smudge : and 

 sometimes the case is reversed, and the figure is the exact nega- 

 tive of the former kind ; that is to say, the area of the figure is 

 indicated by an uniform dark tint, whilst the surrounding stone 

 is unstained." Dr. Sheppard suggests "that an exact pattern 

 seems to have been made in paper and then fixed to the wall 

 whilst it was brushed over with linseed oil. But this does not 

 account for the white effigies on a dark ground." 



I would commend these impressions to the notice of those 

 interested in the subject. It may be that, though some were 

 made intentionally, others are the result of simple contact, 



Mas)n College, Birmingham, February 4. 



F. J. Allen. 



Fossil Plants as Tests of Climate. 



In continuation of my recent letter, permit me to call atten- 

 tion to a communication on the bread fruit trees in North 

 America, by Mr. F. H. Knowlton, of the National Museum, 

 Washington, U.S., which appears in your American contem- 

 porary Science for January 13. The forty living species of 

 Artocarpm are all confined to tropical Asia and the Malay 

 Archipelago. A. incisa, the true bread fruit tree, and one or 

 two others, are largely cultivated in the tropics. They are small 

 Or medium-sized trees with a milky juice, large leathery leaves, 

 and monoecious flowers. The female flowers are long club- 

 shaped spikes, which uniting form one large mass known as the 

 "bread fruit," the interior containing a pulp when ripe like new 

 bread. 



The first fossil bread fruit was discovered in boulder county 

 Colorado in late cretaceous rock, and was named by the late Prof. 

 Le ) Lesquereux iT/>/rzVa (?) Lessigiana, other fragments he called 

 Aralia punge?is. The subsequent researches, or more perfect 

 specimens of Dr. A. S. Nathorst, proved these to belong to one 

 species, Artocarpus Lessigiana. Dr. Nathorst is the discoverer 

 of another species closely allied to A. incisa, which he calls 

 A. Dicksoni, which he obtained from the cretaceous flora of 

 Waigatt, West Greenland, which the previous labours of Profs. 

 Heer and Nordenskiold had shown to be of a tropical or sub- 

 tropical character, containing as it does numerous species of 

 ferns of the order Gleichenialese, and several species of cycas. 

 Chas. E. de Range. 



H.M. Geological. Survey, Alderley Edge, Manchester. 



Lunar Rainbow in the Highlands. 

 This interesting phenomenon (a very unusual one in this 

 latitude) was observed near here on the morning of the srd inst., 

 about six a.m. The moon was two days past full, and was not 



NO. 1215, VOL. 47] 



shining particulaiiy brightly, being obscured, except at consider- 

 able intervals, by driving mist and light clouds. The bow, how- 

 ever, was exceedingly well marked, and formed a singularly 

 beautiful object, stretching as it did completely across the north- 

 western end of Loch Oich, glimmering against the dark back- 

 ground of the mountains, and sinking into the water on the 

 southern shore of the loch. The general colour of the bow was 

 yellow deepening into orange, several of the prismatic colours, 

 however, being intermittently visible, especially a tinge of violet 

 on the upper side. O. S. B. 



The Abbey, Fort- Augustus, N.B. 



OPTICAL CONTINUITY.^ 



Ty" EENNESS of sight is measured by the angular dis- 

 ■'■^ tance apart of two dots when they can only just be 

 distinguished as two, and do not become confused together. 

 It is usually recko.ied that the normal eye is just able or 

 just unable to distinguish points that lie one minute of a 

 degree asunder. Now, one minute of a degree is the 

 angle subtended by two points, separated by the 300th 

 part of an inch, when they are viewed at the ordinary 

 reading distance of one foot from the eye. If, then, a 

 row of fine dots touching one another, each as small as 

 a bead of one 300th part of an inch in diameter, be 

 arranged on the page of a book, they would appear to the 

 ordinary reader to be an extremely fine and continuous 

 line. If the dots be replaced by short cross strokes, the 

 line would look broader, but its apparent continuity would 

 not be affected. It is impossible to draw any line that 

 shall commend itself to the eye as possessing more regu- 

 larity than the image of a succession of dots or cross 

 strokes, 300 to the inch, when viewed at the distance of 

 a foot. Every design, however delicate, that can be drawn 

 with a line of uniform thickness by the best machine or the 

 most consummate artis , admits of being mimicked by the 

 coarsest chain, when it is viewed at such a distance that 

 the angular length of each of its links shall not exceed 

 one minute of a degree. One of the apparently smoothest 

 outlines in nature is that of the horizon of the sea during 

 ordinary weather, although it is formed by waves. The 

 slopes of debris down the sides of distant mountains 

 appear to sweep in beautifully smooth curves, but on 

 reaching those mountains and climbing up the debris, the 

 path may be exceedingly rough. 



The members of an audience sit at such various dis- 

 tances from the lecture table and screen, that it is not 

 possible to illustrate as well as is desirable, the stages 

 through which a row of dots appears to run into a con- 

 tinuous line, as the angular distance between the dots is 

 lessened. I have, however, hung up chains and rows of 

 beads of various degrees of coarseness. Some of these 

 will appear as pure lines to all the audience ; others, 

 whose coarseness of structure is obvious to those who sit 

 nearest, will seem to be pure lines when viewed from the 

 furthest seats. 



Although 300 dots to the inch are required to give the 

 idea of perfect continuity at the distance of one foot, it 

 will shortly be seen that a much smaller number suffices 

 to suggest it. 



The cyclostyle, which is an mstrument used for multiple 

 writing, makes about 140 dots to the inch. The style has a 

 minute spur wheel or roller, instead of a point ; the writing 

 is made on stencil paper, whose surface is covered with a 

 brittle glaze. This is perforated by the teeth of the spur 

 wheel wherever they press against it. The half perforated 

 sheet is then laid on writing paper, and an inked roller is 

 worked over the glaze. The ink passes through the per- 

 forations and soaks through them on to the paper below ; 

 consequently the impression consists entirely of short and 

 irregular cross bars or dots. 



I Extract from a lecture on The Jiist-Perceplible DifTerence," delivered 

 bef.jie the Royal Institution oa Friday, January 27, by Francis Gallon, 

 F.R.S. 



