54 



HA RD WICKE'S SCIENCE- G OS SI P. 



mean focus from D, by different degrees of obliquity 

 given to the incident pencil. But in order to save 

 space, I will take a single case, and explain in what 

 respects it differs from others which might have been 

 presented. 



In Fig. 23 the axis ld of a simple divergent 

 pencil forms an angle of 90 with M D, the axis of the 

 microscope, and of 45 with N D the principal axis of 

 the concave mirror. The distance of L from D is the 

 same as in the last figure. The distance of the mean 

 focus from D is not the same, but is reduced from 

 2*25 inches to 1*4 inch only; it is at the point f 2 

 instead of at/ . The accuracy of the focus has also 

 suffered by the obliquity of the incident pencil. It 

 will be well to compare this figure with the corre- 

 sponding one for parallel rays (Fig. 160, vol. (or 

 1886, p. 250). The important point to be noticed 

 is, that while the parallel rays in Fig. 160 may come 

 either from a lamp at the distance of 2 feet, or from 

 a white cloud at the distance of a mile, without 

 altering the position of the mean focus F 2 , the 

 divergent rays in Fig. 23, on the contrary, cannot be 

 focused fairly at the point / 2 unless they emanate 

 from the point L. The position of the mean focus 

 f 2 depends, therefore, partly upon the angle of 

 incidence and partly upon the distance of L from D, 

 the centre of the mirror ; whereas in the case of the 

 parallel pencil, it depended upon the angle of 

 incidence alone. 



If the angle of incidence were 30 instead of 45 ' 

 and the distance of L from D remained the same, the 

 mean focus would be at the point /', the focal 

 distance being 1 ' 82 inches ; and were the angle of 

 incidence 6o°, the mean focus would be at the point 

 f 3 , the focal distance being 'g inch. In the former 

 case the focus would be much more satisfactory than 

 in the latter. 



For each of these angles of incidence the distance 

 between L and D could be largely varied, and every 

 variation would alter the distance of the conjugate 

 focus. For instance, if l in the figure were moved 

 towards D, f 2 would retreat towards M and would 

 reach/ by the time that L had reached L 1 . When 

 L arrived at L 2 , there would be no conjugate focus at 

 all, the reflected rays having become parallel. If L 

 were moved away from D,/ 2 would approach D until 

 it coincided with the position of F 2 in Fig. 160 (vol. 

 for 1886, p. 250), L having by that time become so 

 distant as to render the incident rays parallel. 



{To be continued.) 



Cage Birds surviving the Winter. — There 

 seems no reason why they should not if the season is 

 mild, and plenty of suitable food. I possessed a 

 canary that escaped and lived. It was seen a year 

 after, and being a hen, it may have mated with a 

 wild species. Tropical birds certainly would not 

 survive. 



HOOKER'S STUDENT'S FLORA AND THE 

 LONDON CATALOGUE. 



I WAS much pleased with the genial tone and 

 kindly spirit of Mr. Wheatcroft's letter in the 

 November issue of Science-Gossip ; but I must ask 

 the privilege of making a few comments upon it. 



If I erred in placing too much reliance upon the 

 accuracy of knowledge and soundness of judgment of 

 Sir J. D. Hooker, I think Mr. Wheatcroft equally 

 and similarly errs in regard to Dr. Asa Gray. The 

 principle of nomenclature laid down by Dr. Gray is 

 undoubtedly good ; and if its application could rest 

 with a man possessed of an infallible and omniscient 

 mind, I would at once chant " Amen" to it. But it 

 is obvious that such condition can never be obtained, 

 the most eminent is fallible, and hence a rigid appli- 

 cation of Dr. Gray's principle is inadmissible. It 

 excludes all hope of revision, no matter how grave 

 may have been the error in the first instance, and, as 

 I conceive it, this exclusion of revision defeats at 

 least one of the fundamental principles of science, 

 viz. the correction and elimination of error, and 

 the substitution for it of a nearer approximation to 

 truth. 



It is unfortunate that Mr. Wheatcroft did not 

 consult Sir J. D. Hooker's " Student's Flora," 

 before fixing upon its author, even by implication, 

 the presumptive blame for the change of name cited 

 as an example. As a matter of fact, Sir J. D. 

 Hooker has not changed the name Chlora perfoliala 

 to Blackstonia perforata, unless he has done so since 

 the publication of the third edition of his "Student's 

 Flora," for in that book the old name is retained. 

 Why it has been changed in the " London Catalogue " 

 I do not pretend to know, perhaps the editor will let 

 us into the secret, not of this only, but also of others ; 

 one thing is certain, that however slight a change be 

 made,- it is sure to clash with custom at some point, 

 and thus affect the convenience of a greater or smaller 

 number of individuals ; and the " London Catalogue " 

 seems to have been altered and made unintelligible to 

 those accustomed to the old edition, as much, if not 

 more, by the re-numbering of the species and intro- 

 duction into the body of the work of many of those 

 excluded species and aliens which in former editions 

 were put at the end, as it has been by changes of 

 names. 



Mr. Wheatcroft disclaims any but the purest motive 

 for his writing on this subject ; I have no wish to 

 question his sincerity nor to impute to him other 

 than the motive he has suggested. 



In the hope of helping such of my botanical 

 brethren as do not possess the new edition of the 

 " Student's Flora," I append a list of the changes 

 of names, and some alterations of classification which 

 appear in it as compared with the first edition. It 

 will be observed that in nearly every case where a 

 change has been made, the old name is retained as 



