June 27, 1878] 



NATURE 



221 



some months past, a fact of which the members of the British 

 Association may take advantage this year. It is now Scotland's 

 turn. W. H. C. 



Ophrys muscifera 



On the afternoon of June 2, 1878, I observed some new 

 facts, which, I think, are of importance in elucidating the 

 hitherto mysterious fertilisation of the Fly -Orchis. In sunny 

 weather and under normal conditions the labellum secretes fluid, 

 and a broad central longitudinal stripe of its surface is covered 

 with small drops. Of fifty fresh flowers I found the labellum 

 in thirteen covered with drops, in twenty-five shining with 

 adhering moisture, in twelve without any conspicuous trace of 

 fluid. The two small shining projections on each side of the 

 base of the labellum (the sham-nectaries of Sprengel) were quite 

 di-y in all the flowers. In one flower I saw a fly (Sarco- 

 phaga sp.) sitting on the labellum and licking the drops. Its 

 head was directed towards the base of the labellum. On my 

 approaching it flew away before having reached the sham- 

 nectaries, and the flower visited by it was found without pollen 

 on the stigmas, and with both pollinia in their cells. Never- 

 theless, it is most probable that this fly, if not disturbed by my 

 approach, would have stepped forward on the labellum, and, 

 trying one of the sham-nectaries, would have removed one of 

 the pollinia and perhaps transferred to the stigma of another 

 stem, in the manner described by Charles Darwin (" Fertilisation 

 of Orchids," p. 47). 



For observing the fluid secreted by the labellimi it may be 

 essential to examine plants in their native habitats, not plucked 

 ones. Hermann Muller 



Lippstadt 



The Jura 



In the midst of the enjoyment of quiet and beautiful scenery I 

 cannot refrain from writing, in the interest of geology, to attract 

 attention to the facilities for the study of the Jura range afforded 

 by a railway recently opened from 13ale, vid Delemont and the 

 Miinster Thai, to Bienne. It crosses the range at, relatively to 

 the anticlinal, a considerable angle, necessitating no less, as I 

 am told, than twenty-five tunnels great and small (I did not count 

 them myself). 



Consequently, in a short morning's railway ride the traveller 

 sees a vast deal of Jurassic structure, added to which the Miinster 

 Thai, formerly a rather tiring day and a half's drive, is replete 

 with rock, forest, and pasture scenery of very great beauty. 



Travellers thus crossing the Jura on their way to the Alps and 

 returning from Lausanne by Vallorbes to Paris, will thank me, 

 I think, for pointing out what, if only from a scientific point of 

 view, are two recently-developed routes, far more interesting 

 than the customary approaches to this land of wonders. I 

 repress poetic and mountaineering sympathies. 



Pension Mounoud, Veytaux-CMllon, MARSHALL Hall 



Canton Vaud, Jime 21 



THE TRANSIT OF VENUS PHOTOGRAPHS^ 



THE photographs which have been measured were 

 taken with the five photoheliographs made by Mr. 

 Dallmeyer for the Transit of Venus expeditions, on 

 " patent plates " 6 inches square, the images of the sun 

 being very nearly 3*9 inches in diameter. The dry pro- 

 cess of Capt. Abney was used throughout. 



The measuring instrument, the determination of the 

 errors of its glass millimeter scale, and the method of 

 obtaining the optical distortion of the photoheliographs, 

 hare already been described in the Society's Proceedings. 

 It has been found by an elaborate investigation that the 

 lines of equal distortion were sensibly circles concentric 

 with the centre of the field. The actual correction for 

 distortion for that zone of the field in the points to be 

 measured generally fell, was exhibited on the board, and 

 was almost identical for all five instruments. 



Before commencing the measures of a negative, the 

 position of the line of centres was marked upon the film 

 by a simple mechanical process. This operation has 

 been performed independently by Mr. Burton and myself^ 



' Paper read by Capt. Tupman at the meeting of the R.A.S. on June 14, 

 on the measurements of the Transit of Venus photographs. 



with no sensible difference. I have paid no attention to 

 the marks left by Mr. Burton on the plates, and found 

 that my own coincided with them in direction. 



In placing the negative in the instrument the circular 

 carrier was turned about until the line of centres was 

 truly parallel to the direction of the sliding motion of the 

 microscopes. 



When the negatives are placed under the microscope 

 with an amplification of only five or six diameters, the 

 limbs of both planet and sun, even those which are pretty 

 sharp to the unaided eye, become extremely indistinct, 

 and the act of bisecting a limb with the wire or cross of 

 the micrometer is mere guess-work. The deposit of 

 silver fades off gradually to nothing, and the denser the 

 film the broader generally is the zone of fading off and 

 the more uncertain the measures. In many cases the 

 difficulty is aggravated by ruggedness due to atmospheric 

 disturbances, but the smooth and gradual fading offis the 

 chief cause of uncertainty. 



There is only one really sharp picture in the whole col- 

 lection, including the Indian and Australian contingents, 

 and that is one of Capt. Waterhouse' s wet plates, taken 

 at Roorkee with a Dallmeyer instrument precisely similar 

 to the others. 



It should be remarked that in these instruments the 

 artist has attempted to unite the photographic and visual 

 foci on the collodion film. No doubt some sharpness of 

 the photographic image was thus sacrificed, but this has 

 little or nothing to do with the unfortunate failure of the 

 photography generally. 



Each photograph has been measured six times by Mr, 

 Burton and six times by myself. I am not able to include 

 in my series of measures all the photographs measured 

 by Mr. Burton, for the reason that when some of them 

 Avere viewed through the microscope I could see nothing 

 to bisect, either from the extreme faintness of the film, 

 or from its too gradual fading off. 



Mr. Burton generally employed a cross of webs, but I 

 have preferred a single very fine web, the breadth of 

 which was eliminated in the mean by the mode of 

 bisecting. 



It had been suggested that the measuring instrument 

 should possess the power of rotating the sun's image 

 about a mechanical centre. This would be useful in 

 some cases of rugged limbs when the sun' s image was 

 not rendered elliptical by refraction, but in my opinion 

 would make no material difference in the accuracy of 

 measurement. The rotation could only be applied to the 

 limbs of the sun, whereas, perhaps, the greatest difficulty 

 had been at the limbs of the planet. 



From the measures, corrected for distortion, were 

 obtained the 'photographic diameters of the sun and of 

 Vefitisj the former presumably enlarged, the latter dimi- 

 nished by irradiation in a sensibly equal degree. The 

 sum of the measured diameters in millimetres was com- 

 pared with the sum of the tabular diameters, subject to 

 errors, for the scale value, and thus every photograph 

 furnished its own scale. 



The measured distance of centres affected by errors of 

 semi-diameter was then compared with the tabular dis- 

 tance affected by errors of parallax, right ascension, and 

 north polar distance. From each photograph was formed 

 an equation involving all the unknown quantities, of which 

 the errors of parallax and of semi-diameters were the more 

 important. 



The rigorous solution of the equations resulting front 

 Mr. Burton's measures is. 



Mean solar parallax = 8"*i65 — '209 (<//* + dr] 

 „ d R.A, ... = -f 5-38 -1- -287 {dR 4- dr) 

 „ rf N.P.D. ... =- 5-10 - -882 (^i?-^r). 



The parallax deduced being absurdly small— altogether 

 inadmissible, indeed — the Astronomer-Royal suggested 

 that the quantity {dR-\-dr), or the sum of the corrections 

 to the tabular semi-diameters, should be considered the 



k 



