436 



NATURE 



[December 19, 1912 



two minors being adjustably clamped together, so 

 that they can be set exactly at right angles to one 

 another. Now let them be placed so that their line 

 of contact is inclined 45° to the horizon, while (in a 

 roughly approximate sense) the vertical plane through 

 that line of contact bisects externally the angle be- 

 tween the mirrors. Vertical lines imaged by suc- 

 cessive reflection at the two mirrors will thus appear 

 horizontal, and conversely. 



In the case of an echelon grating, the train would 

 bear a general resemblance to a Littrow spectroscope. 

 The light would pass successively through a slit 

 (shortened to a minute square), an objective, and an 

 echelon, and after reflection at the mirrors would 

 return through the echelon and objective, and be 

 brought to a focus in the plane of the slit. For the 

 full advantage of crossed dispersions to be thus 

 realised, it is, of course, essential that the elifective 

 aperture of the echelon should be at least as high as 

 it is wide, the width being measured parallel to the 

 dispersion. In echelon gratings and Lummer-Gehrcke 

 plates this generally holds good, though in many 

 gratings the length of the rulings is insufficient for 

 the corresponding condition to be satisfied. 



The pair of mirrors described might be replaced 

 by an accurately right-angled prism, with reflecting 

 faces meeting in as sharp and clean a line as possible. 



The suggested arrangement may be modified by 

 allowing the beam to pass through a second objective 

 and be brought to a focus in the usual way. A small 

 right-angled prism can then be used to return the 

 beam through the lenses and the echelon between 

 them, and since the intersection of its reflecting faces 

 should lie strictly in the plane of the first formed 

 (singly dispersed) spectrum, it is easy to arrange so 

 that this intersection, as finally viewed, is to one side 

 of the useful field- In thise case the prism need not 

 bo accurately right-angled, nor indeed is any great 

 demand made on its other optical qualities; it mav be 

 some set-off against this that four transmissions 

 through object-glasses are involved. 



If an echelon grating of reflecting type is to be 

 crossed with its own dispersion, a method essentially 

 similar to the last-mentioned modification can be used. 

 The apparatus, as arranged for single dispersion, 

 having been auto-collimated, the beam would in the 

 present case be twice brought to a focus, and would 

 in all pass four times through one and the same 

 obiective. C. V Burton. 



Boar's Hill, Oxford, December 7. 



Petrifactions of the Eariiest European Angiosperms. 



Until the three specimens from the English Aptian 

 in the British Museum were recognised as Angio- 

 sperms and described in my paper (Phil. Trans. Roy. 

 Soc, series B, vol. cciii., pp. 75-100, plates v-viii, and 

 kindly reviewed in Nature, August 22, p. 641), Angio- 

 sperms were supposed not to have existed in northern 

 Europe at that early date. Those three specimens 

 came from two different localities, which minimised 

 the chances of error, but it is highly satisfactory to 

 have to record the discovery of another specimen from 

 a new locality. 



The new specimen is from the Lower Greensand of 

 Kent, and belongs to the Maidstone Museum. While 

 pursuing my study of the Lower Cretaceous flora I 

 recently visited the Maidstone Museum, which has the 

 best extant collection of Lower Greensand fossil plants 

 from Mr. W. H. Bensted's famous Iguanodon Quarry. 

 The collection includes a number of large pieces of 

 silicified wood from other of the numerous quarries 

 in the Lower Greensand in the district. All these 

 I examined carefully, and the majority of pieces 



NO. 2251, VOL. go] 



proved to be Gymnospermic, but one of the large bits 

 of petrified wood arrested attention. Mr. Allchin, the 

 present curator of the museum, generously allowed 

 me to have sections cut from it, w'hich prove the 

 specimen to be a portion of the trunk of a large 

 Vvfoody Angiosperm. -A detailed and illustrated account 

 of its anatomv will follow in due course, but it may 

 be remarked here that its general characters differ 

 from those of the three other described species from 

 this horizon, and it certainly represents a new species 

 and possibly a new- genus. 



As the question of the origin of Angiosperms is one 

 in the forefront of controversy at present, and is one, 

 moreover, about which we have so remarkably little 

 evidence, the discovery of this, which is only the 

 fourth specimen of Aptian Angiosperms yet obtained 

 from northern Europe, is satisfactory in confirming 

 the conclusions reached from the study of the three 

 British Museum specimens. Marie C. Stopes. 



Smoke Trace of Compound Vibrations of Tuning-fork. 



I READ with interest the note by Mr. F. H. Parker 

 on upper partials of a tuning-forl-c, which appeared in 

 Nature of November 28, p. 361. 



As an alternative to taking the first upper partial 

 to be 66 times the frequency of the prime, or con- 

 firming the relation by separate traces, may I suggest 

 the plan of making a trace of the vibration com- 

 pounded of the 

 two? 



The accompany- 

 panying print is 

 from one corner 

 of a smoke trace 

 used by me at a 

 popular lecture in 

 1901. One curve 

 shows the funda- 

 mental (128 per 

 second), another 

 the first upper 

 partial, while the 

 centre curve of 

 the three shows 

 the form of vibra- 

 t i o n executed 

 when the first 

 upper partial is 

 sounding, together 

 with the prime. 

 The three sounds 

 may be heard by the audience, and the smoke traces 

 of each obtained in their presence, and then projected 

 by the lantern. The compound vibration is easilv 

 obtained by striking the fork on a hard surface, such 

 as a counter, and so presents no difficulty whatever. 

 The ratio of frequencies of first upper partial and 

 prime for the ratiier slender fork in question is seen 

 to be of the order 6'25. E. H. Barton. 



University College, Nottingham, December 7. 



BREATH FIGURES. 

 AT intervals during the past year I have tried 

 ■^*- a g'ood many experiments in the hope of 

 throwing further light upon the origin of these 

 figures, especially those due to the passage of a 

 small blow-pipe flame, or of hot sulphuric acid, 

 across the surface of a glass plate on which, before 

 treatment, the breath deposits evenly. The even 



