270 



HARDWICKE'S SCIENCE- GOSSIP. 



[Dec. 1, 1868. 



inch, and that in a space only one half as great in 

 the coarsest band of the Nobert plate there are 

 seven lines, and in the finest band there are thirty- 

 five liues." Another of these plates is ruled with 

 nineteen bands of lines, in the first of which the 

 lines are one thousandth of a Paris line apart, while 

 in the finest or nineteenth band the lines are only 

 the ten thousandth part of a Paris line apart. It 

 may well be asked whether human art has ever 

 made au instrument capable of rendering visible 

 lines in such close contact with each other. The 

 problem is exactly the parallel of that of the power 

 of the telescope of separating double stars. 



The lines are ruled on the under surface of a thin 

 glass cover, which is cemented at the edges with 

 Canada balsam to a glass slide, on which the frac- 

 tions of a Paris line corresponding to the principal 

 lines are written with a diamond. Eor the purpose 

 of testing the resolving power of the microscope 

 these lines are admitted to be the best known tests, 

 not only in consequence of their exceeding fineness, 

 but also because they are ruled to a known scale. 

 Up to the present time the finer bands of lines in 

 these plates have never been resolved, and have 

 therefore never been seen even by the maker of 

 them, though there can be no reasonable doubt that 

 the lines do exist. Mr. Stodder professes to have 

 seen the nineteenth baud satisfactorily, in which the 

 lines are one ten thousandth of a Paris line or one 

 hundred and twelve thousandth of an inch apart. 

 But a later writer in the Quarterly Journal of 

 Microscopical Science says, that his own observations 

 have convinced him that Mr. Stodder saw spurious 

 and not real lines. Eor in the finer groups of lines 

 it appears that when the resolving power of the 

 objective approaches its limit spectral lines are seeu ; 

 that is, lines composed of two or more real lines, 

 and the true lines cannot be seen with any degree 

 of certainty. The difficulty of counting the lines is 

 of course much greater than that of merely seeing 

 them, just as it would be extremely difficult to count 

 the pickets in a fence at the distance of a hundred 

 or an hundred and fifty yards, though the pickets 

 themselves may be distinctly visible. In the micro- 

 scope it is true that the micrometer is an aid in 

 counting the lines ; but in counting lines of such 

 exquisite fineness either the micrometer or the stage 

 must be moved, and it is almost impossible to con- 

 struct an apparatus that can be moved at once the 

 hundred thousandth part of an inch and no more ; 

 while any change necessitates a slight change of 

 focal adjustment, and then it is extremely difficult 

 to fix the exact line last counted. Colonel Wood- 

 ward has described a method by which the Nobert' s 

 lines may be counted without photographing them : 

 " We set up the microscope as though to take a 

 photograph, remove the eye-piece, receive the image 

 on a piece of plate-glass and view it with a focus- 

 ing glass, on the field-lens of which a black point is 



remarked. As the focusing glass is moved on the 

 plate from side to side, the black point is moved 

 from line to line. The lines may thus be counted 

 with as much ease and precision as though they 

 were large enough to be touched by the finger." 



Still Colonel Woodward does not seem to have got 

 beyond the resolution of the fifteenth band of the 

 nineteenth band plate, and which corresponds in 

 fineness with the last band of the thirty band plate, 

 in which the lines are the eight thousandth part of 

 a Paris line apart, or the ninety thousandth part of 

 an inch. This band of lines is somewhat finer than 

 the closest bines upon the valves of Pleurosigma as 

 given in Dr. Carpenter's work. His table, taken 

 from Smith's monograph on the Diatornaceae, gives 

 the average distances of the lineatiou of Navicula 

 rhomboides aud Navicula sigmoidea at the eighty- 

 five thousandth part of an inch. So that the 

 hundred thousandth part of an inch seems to be 

 quite the limit of our present microscopic powers. 



R. H. N. B. 



SURREY BLACKBERRIES. 



\\7~HILE spending a few days in the neighbour- 

 ™ * hood of Ockham, Surrey, in the absence of 

 anything else of particular notice in the natural 

 vegetation of the district, I was led, more by the 

 abundance of the fruit than anything else, to closely 

 observe the different varieties of the genus Rubus. 

 The prevailing form in this part of the country is 

 that known as Rubus discolor, a very common Surrey 

 plant, and very distinct, with a dark-green upper 

 surface to the leaves, and a white hairy under sur- 

 face, hence the two distinct colours, which give it 

 its specific name. This may be further distinguished 

 by its black fruit shining with a metallic lustre, and 

 the regular size and rounded form of each individual 

 drupel, the fruit of the typical form of this plant 

 is hard and not very good in flavour ; but there are 

 several varieties of it seemingly from crossing with 

 Rubus corylifolia, which is a decided improvement 

 as far as flavour is concerned. These varieties are 

 recognized by the foliage differing in a greater or 

 lesser degree from the typical form. The former 

 has the leaves of the carpinifolia sections of the 

 genus, while the varieties have a broader character 

 and a fuller margin, bringing them nearer to the 

 corylifolia section; these seem to be heavier croppers, 

 and if ever blackberries become cultivated, these 

 varieties would be the most useful on account of the 

 small space they occupy. Two other varieties 

 occurred sparingly, which I do not remember having 

 noticed previously, they were tall growing kinds, 

 the most striking character being the very large 

 size of their fruit, the drupels, in both varieties, be- 

 ing twice as large as the sorts previously observed, 

 and both of them also agreed in passing rapidly 

 from a pale pink to black ; but the difference in the 



