6l2 



NA TURE 



[October 27, 1892 



(rt) The colour (orange) is due to pigment in the skin, and 

 not to the red colour of the nervous system ; I may 

 mention that Duges' species, "■Prostoma clefisinoides," was 

 yellow ochre, and "/V. lumbricoideum" was yellow 

 marbled with red; whilst Leidy's *' Emea rubra" via.% 

 yellowish flesh-coloured (probably due to the haemoglobin 

 in the nervous system). 

 (3) The anterior pair of eye-spots is further from the 

 prostomium than in Silliman's drawing ; I found no third 

 pair of eye-spots, which, however, it is stated, is absent 

 in the young. 

 {c) The ciliated pits are further forward, being midway 



between the brain and the anterior end of the body. 

 {d) The proboscis and its retractor muscle are much more 

 undulating, when withdrawn into the body, than Silliman 

 shows. 

 The proboscidial spine, with its groups of accessory spines, 

 agrees very closely with the figures given by Silliman. 



I can say nothing about the generative organs. For the 

 present, then, I must leave undecided the specific name of this 

 British Tetrastemma. W. Blaxland Benham. 



Anatomical Department, Museum, Oxford, 

 Oct. 12. 



Protective Mimicry. 



Mr. Bateson's letter on "Aggressive Mimicry " (Nature, 

 October 20) recalls to my mind a curious case of protective 

 mimicry which came under my notice last August on Dart- 

 moor. Large patches of the heath had been burnt, a common 

 practice on the moorlands to ensure a fresh young growth 

 for the sheep. The whole ground was alive with a common 

 species of orthoptera (Locustina), the small green grasshopper 

 with short antennae. They leapt aside at every step in the short 

 grass and scrubliy heath ; upon the burnt patches they were 

 equally numerous, but with this diff"erence — all, without excep- 

 tion, were coal black on abdomen, thorax, and head, whilst the 

 wings were of an ashen hue. So much did the colour adapta- 

 tion resemble the blackened turf and heath they hopped amongst 

 it was almost impossible to follow them with the eye ; we made 

 many amusing attempts, but were nearly always defeated. I 

 measured one of these burnt patches, and found it to be from 

 thirty to forty yards square. A yard or two from this, on the 

 untouched herbage all the Locustina were bright green. I found 

 one specimen on the borderland in a transition state, not dull 

 all over as I had expected, but in spots and patches of bright 

 green and black. One enemy at least of these insects 

 abounded on the moor, namely, the common lizard {Zootcea 

 vivipard), for I have observed there is no food lizards will eat 

 more greedily than grasshoppers. I have seen some that I 

 have in captivity swallow twenty or thirty in two or three 

 minutes, even after their usual meal of worms. They always 

 beeame greatly excited, if one may apply so warm an expression 

 to such cold-blooded animals, and rushed about the case when 

 a collection of live grasshoppers were thrown to them. Certainly 

 I was much struck by the rapid action of the power possessed 

 by these Locustina on Dartmoor of assimilation to environ- 

 ment, and did not doubt but that this colour adaptation was for 

 the purpose of protection, the eye producing by reflex action the 

 change in the pigment cells. Rose Haig Thomas. 



STELLAR PARALLAX} 



T^HE Delegates of the University Press have recently 

 -*• published the results of Prof. Prltchard's 

 systematic investigations into the parallax of those stars 

 of the second magnitude whose North declination per- 

 mits the inquiry to be made with facility and advantage 

 in these latitudes. Our first feeling on glancing over the 

 contents of this brochure must be one of hearty con- 

 gratulation to the distinguished professor that he has 

 been permitted to see the full outcome of a protracted 

 inquiry, conducted at a period in his life when a less 

 energetic astronomer would have felt himself justified in 

 withdrawing from active participation in scientific research. 



I "Researches in Stellar Parallax by the aid of Photography." By 

 Charles Pritchard, D.D., F.R.S., Savilian Professor of Astronomy in 

 Oxford. 



NO. 1200, VOL. 46] 



Prof. Pritchard might well have been content to rest or» 

 the laurels he had won, and to have staked his reputation 

 upon that career of acknowledged utility which has 

 marked his direction of the Oxford University Obser- 

 vatory. 



Immediately on the completion of the photometrical 

 examination of Argelander's Uranometria, and with a 

 zeal that admitted of no delay, Professor Pritchard 

 busied himself with this inquiry into the parallax of 

 stars of the second magnitude. But if the inquiry was 

 undertaken with eagerness, and pursued with ardour and 

 resolution, it was not characterized by hurry, or its suc- 

 cess imperilled by incompleteness. Confident himself 

 that photographic methods possessed the requisite 

 accuracy to make the research successful and trust- 

 worthy, the Savilian Professor set to work to establish the 

 reliable character of measurements made on sensitized 

 films, and not till that confidence was demonstrated did 

 he embark upon the larger work now under notice. These 

 preliminary inquiries have been published in a series of 

 papers in the proceedings of the Royal and Royal 

 Astronomical Societies, and the confidence gradually 

 acquired by enlarged experience induced him to 

 proceed with the determination of the parallax 

 of 61 Cygni, the results of which are published 

 in detail in the third fasciculus of the Annals 

 of the University Observatory. In this case he selected 

 four stars in the immediate neighbourhood of the princi- 

 pal star, and sought the difference of parallax between 

 each of the components and of the four stars of compari- 

 son. This long research may be regarded by some as a 

 work of supererogation, inasmuch as the labours of 

 Bessel and that of many later astronomers have satis- 

 factorily settled the parallax of this star within very 

 approximate limits. But if we properly understand the 

 motives of Prof. Pritchard, his intention was not so much 

 I to seek anew the parallax of that system, as to discover 

 I with what degree of accuracy the method of photography, 

 I hitherto unapplied in this direction, represented the work 

 of others made directly in the field of the telescope. Nor 

 was this his only view. By selecting four stars in the 

 ' immediate neighbourhood of 61 Cygni and seeking the 

 difference of parallax between these stars of comparison 

 and each of the components of the system, he instituted 

 I a very severe inquiry as to the trustworthiness of that 

 method, which he had imagined as capable of dealing 

 with the delicate question of stellar parallax. The 

 severity of the test consists in deducing the same 

 '■ value of the parallax (eight in all) from each set of 

 ; measures, and as a matter of fact the accordance, inter 

 sc between these several determinations is as close as 

 I could have been anticipated, and likewise in satisfactory 

 j unison with the work of other astronomers. 



The completeness of this inquiry and the publication of 

 ! it in detail have had two happy results. In the first 

 ■ place, Prof Pritchard has, in the present instance, been 

 able to confine the printing within very narrow limits, so 

 narrow, indeed, as possibly not to have done himself 

 ; justice. The details of his process, the mutual agreement 

 of his measures, and his method of discussion having all 

 ! been fully set out in his previous work, he has not felt 

 himself obliged to enter into these minute particulars, 

 but has contented himself with presenting the results. 

 This method of arrangement, no doubt suggested in the 

 first place by economical motives, has afforded op- 

 portunity for adding a very interesting history of 

 the processes and results that have hitherto been 

 followed with more or less success by others, and 

 also the exhibition in a concise form of the different 

 values of the more trustworthy determinations, derived 

 by previous observers. The second advantage, imme- 

 diately arising from the earlier investigations, is, that an 

 examination of those results has shown that no increase 

 j of accuracy (commensurate with the increased labour at 



