December 15, 1892J 



NA TURE 



161 



Another objection is that boats of British nationality are not 

 the only ones engaged in the small fish trade, and it is true that 

 during the summer months a number of German, Dutch, and 

 Danish boats are occupied in catching small plaice. But they 

 are all of small tonnage, some of them only open boats ; and I 

 understand that from the manner in which the trawl is 

 handled by German and Danish boats no injury is done to the 

 unmarketable fish, whilst the saleable part of the catch appears 

 to be exported chiefly to London. Hence the proposed mea- 

 sures of prohibition would give no advantage to these nations. 

 The German steam trawlers, according to my information, do 

 not molest the small plaice at all. Of the proceedings of the 

 Dutch bombs I have little knowledge, but from the small size 

 of their gear, their share in the destruction cannot be a very 

 large one. Foreign- caught fish, except Norwegian salmon and 

 mackerel and Dutch soles, including only a small percentage of 

 undersized fish, rarely come to the Grimsby market, but on two 

 occasions large consignments of small plaice, comprising, as I 

 compute, some 31,000 fish, were sent from Denmark, and 

 recently a consignment of turbot has arrived from Norway. 

 These last fish were about 300 in number, all undersized, viz. 

 from gi to 17 inches, whilst 4 were only from 8 to 9 inches. 

 This is the only instance which has come under my notice of 

 any considerable number of turbot less than 12 inches being 

 present in the market, and, as we have seen, our own fishermen 

 were not concerned in it. 



The last and perhaps the most important objection arises from 

 the difficulty in allowing for that variation in the size of fish of 

 the same species on different parts of our own coast to which 

 Mr. Calderwood alluded in the last number of the Journal, 

 p. 208. The impossibility of utilizing a uniform size limit for 

 all districts sufficiently exemplified by the limit of il inches for 

 the plaice proposed by the Conference of last February, which 

 was the result of a compromise between the trade representatives 

 of the North Sea and south and west coast districts. While 

 perhaps unnecessarily high for the Plymouth district, we have 

 seen that it is altogether too small for the North Sea. The 

 difficulty of having different limits, of local application, will 

 only be felt at such a central port or market as London, to 

 which fish are brought, whether by rail or sea, from all districts, 

 but with proper organization the obstacle does not seem insu- 

 perable. It is conceivable that the law might be evaded by 

 running cutters Irom boats fishing in one district to the parts of 

 another, where the limit was lower, but it is little likely that 

 the firms which are in a position to undertake them, would lend 

 themselves to such operations. There is not the slightest reason 

 to apprehend a general conspiracy of evasion amongst the fisher- 

 men, and the boats which respected the law would form a more 

 efficient police than all the cruisers in the navy, so far as one 

 may judge by the conditions on the Scotch coast, where con- 

 victions of trawlers for infringement of the territorial restriction 

 are frequently secured by the evidence of local line fishermen. 



I must leave to others, who are acquainted with the local 

 conditions, to decide whether the imposition of a size limit is 

 desirable in other districts, but for the North Sea I have not the 

 slightest hesitation in recommending this method of legislation, 

 in the terms I have proposed above, as cheaper and likely to be 

 infinitely more efficacious than any other that can be devised in 

 maintaining the supply of the more important kinds of flat-fish. 

 I need hardly observe that its application to the halibut, which 

 is chiefly a line fish, could not fail to be beneficial to that species, 

 since there is no question but that fish caught on the hook will 

 usually survive if returned ; ' but I do not think that the limit 

 need be as high as the biological one, owing to the difference 

 in the conditions of the trawl and line fisheries. 



I am not prepared to enter at present into the question of 

 mesh legislation, beyond pointing out that it appears to be the 

 only method by which the destruction of immature round fish, 

 notably haddock and whiting, can be checked, since these species 

 are fatally injured by being caught in the trawl, and would not 

 survive if returned. Any great enlargement of the mesh does 

 not appear advisable, since it would afford an opportunity of 

 escape to the mature sole, of which that active species would be 

 extremely likely to avail itself. The remedy seems to lie rather 

 in an alteration of the arrangement of the meshes in the cod- 

 ends, so as to prevent them from closing. On this subject I have 

 been making investigations, but they are not yet sufficiently 

 complete to yield reliable deductions. It is sufficiently evident, 

 as has often been pointed out, that the great breadth of some of 

 ' Except fisli with air-bladders, caught at considerable depths. 



NO. I 207. VOL. 4.7I 



the flat-fish render it impossible to deal with the whole question 

 by restrictions of mesh alone. 



The last matter with which I have to deal is the destruction 

 of very small fish by shove-net and shrimp " seines." If it 

 were only possible to induce the men to cull out the small fish 

 in the water they would do no harm at all, and practically I 

 suppose that, as matters are, they do not greatly injure any 

 species of known value except the plaice, although the small 

 number of sole, turbot, and brill destroyed may represent, from 

 the relative scarcity of these species, a more considerable injury 

 than one would suppose. When fishing by day the shove-net 

 men usually return the fish to the sea, but by night this is im- 

 possible, and the seine men do not seem to make any effort in 

 that direction either by day or night. 



It is a difficult question to deal with, since the shrimp appears 

 to be almost a necessity to some people ; at the same time the 

 small plaice which are destroyed must represent an infinitely 

 greater value than the shrimps. If hatcheries were established, 

 and young turbot, brill, sole, and plaice were enlarged after 

 they had been reared through the delicate larval and metamor- 

 phosing stages, it is reasonable to suppose that they would be 

 conveyed or would find their way to the sandy margins, which 

 seem best adapted to the succeeding stages of their life-history, 

 only to fall into the net of the shrimper. 



I should say that to prohibit the use of any sort of shore 

 shrimp nets during night-time would be a beneficial measure, 

 but there is perhaps sufficient reason for abolishing the industry 

 altogether. Those engaged in it might be sufficiently compen- 

 sated at a moderate expenditure, if indeed it be not contrary to 

 public policy to admit the existence of a vested interest in an 

 occupation which is essentially injurious to industries affecting a 

 much greater section of the community. 



THE NEW TELEPHOTOGRAPHIC LENS. 



T N a small pamphlet of thirty pages, written and published by 

 Mr. T. K, Dallmeyer, the author brings together the 

 various notices bearing on the subject of his new telephoto- 

 graphic lens that have appeared during the last twelve months. 

 He also gives an account of the " simple " and " compound " 

 telephotographic lens, with general instructions for their use, 

 including tables of their properties, and a table showing the 

 diameters of circles of illumination necessary to cover the various 

 sized plates used at the present day. 



The telephotographic lens is, we may say, the latest advance 

 made in the science of optics as applied to photography. By it 

 we are now able to obtain large pictures of animate things situ- 

 ated at long distances with short exposure. In this invention 

 Mr. Dallmeyer has produced a useful, and what may prove a 

 valuable, instrument, and he has opened up quite a new horizon 

 which will not suffer from lack of workers. 



Hitherto the principle involved in the apparatus for the pro- 

 duction of large images consisted first in obtaining the primary 

 image, and second, in subjecting this image to the process 

 of enlargement. To obtain the former a concave mirror, or 

 more generally double convex lens, has been employed, while 

 the subsequent magnification has been produced by placing a 

 secondary magnifier or second positive lens behind the plane of 

 the primary image. 



This method, except in the case of astronomical work, has 

 not been, we may say, popularly used, for the cumbrousness 

 of the apparatus required, and the length of time necessary 

 for exposure have quite prohibited its use for anything but in- 

 animate snbjects. 



It is well known that the focal length of a lens is measured 

 for practical purposes from the principal plane passing through one 

 of the nodal points nearest the principal focal plane to that plane : 

 in most lens-constructions this nodal point lies within the lens- 

 mount. Now it will be seen that if this nodal point could be 

 thrown in front of the lens, that is, on that side away from the 

 focus, the focal length, if measured from the lens, would be 

 shorter. This is exactly what Mr. Dallmeyer has done. In the 

 simple telephotographic lens the anterior element, which is of 

 large aperture and short focus, is a positive lens, while the pos- 

 terior is negative, and of a fractional part of the focal length of 

 the former lens. A diagram showing the lenses in position and 

 the path of a ray of light remind one at first sight of the prin- 

 ciple of the Galilean telescope, with this difference, that the rays 

 emeiging are not divergent, but convergent. In the construction 



