October 27, 1892] 



NA TURH 



625 



u face only pjriodicilly, in one or a few months of the year. 



His is probably dependent upon conditions of reproduction and 



velopment, as well as upon the temperature of the season. 



le daily variations are conditioned by the weather, and par- 



L.cularly by the wind and rain. A shower will very quickly 



reduce the specific gravity of the surface water and thus drive 



the surface-dwelling animals below. Many animals rise to the 



surface only at a definite time of day, some in the morning, 



others at noon, and yet others only towards evening. 



Climatic Diff'erence. — Prof. Haeckel thinks that the quantity 

 of the Plankton is very little dependent upon the climatic 

 ilifference of the zmes, but that the quality is greatly so, and 

 indeed in this way, that the number of component species 

 diminishes from the equator to the poles. These conditions, 

 he believes, are directly referable to the influence of the sun, 

 " the omnipotent creator," whose more direct rays bring about 

 an acceleration in the processes which make up the cycle of life. 

 As this is true of the terrestrial fauna and flora so it is true of 

 the marine. 



Currentic Differences. — Conspicuous differences are also 

 brought about by the numberless currents, great and small, by 

 tlie little-known deep sea ^oceanic currents as well as by the 

 better known great surface currents, the Gulf Stream, the Falk- 

 land Stream, the Guinea Stream and others. These currents 

 play a great rS'e in the distribution of many forms of life. More 

 local influences are exerted by the small currents whose causes 

 are found in the climatic and geographical conditions of the ad- 

 jacent coast. The relations of Plankton life to currents is little 

 known, and needs investigation, but first abetter knowledge of 

 the currents themselves is necessary. 



Almost every one who has seen the surface of the ocean in a 

 calm has noticed the glassy areas of irregular shape. These are 

 found on the high seas as well as in sheltered bays and harbours, 

 and are of very special interest to the student of marine life. 

 So far as made out they are extremely irregular in time and 

 place of appearance, and the cmdiiions governing them have 

 not been carefully studied. They are in a measure influ- 

 enced by winds and currents, by the ebb and flow of the tide. 

 Here, into a limited space, are crowded great numbers of organic 

 forms ; this space is readily distinguished from the surrounding 

 water in which there is comparatively little life. These pheno- 

 mena have been noticed by seafaring men and have many differ- 

 ent names in different countries. 



A word in conclusion as to the bearing and importance of the 

 Plankton in human economy in the near future. When Malthus 

 promulgated his famous doctrine he failed to consider the final 

 element which enters into the problem of human population, 

 the human mmd. The ingenuity of the human mind has 

 tirouwht about a decreased efficiency in the natural checks to un- 

 due increase, and thus an artificial increase in the food supply 

 is rendered necessary for the crowding population. This food 

 supply is now irainly derived from the cultivation of the land. 

 A still further mcrease of population will necessitate a levy upon 

 marine life. As soon as man to any great degree becomes a 

 factor in the Plankton conditions by drawing from it large quan- 

 tities of food, particularly in the form of mature animals, the 

 equilibrium of oceanic life will be disturbed, and must be adjusted 

 by artificial means. But further, a study of the phenomena 

 >! marine life shows that the water as well as the land, through 

 ivation, is capable of producing a greatly increased food supply 

 man. The necessity of cultivating the marine resources is 

 - -n now apparent, and many governments have already begun 

 to cope with the question by the establishment of commissions 

 of fisheries Of these commissions that of the United States 

 stands in the front rank by virtue of its positive results. But io 

 the near future n lividual attention must be turned to supple- 

 menting the terrestrial resources, the wheat fields, the cattle and 

 sheep ranches, by an increasing utilization and development of 

 the possi 'ilities of marine farming ; by fish propagation, by plan- 

 tations of oysters, clams, quahaugs and scallops, by raising herds 

 lobsters and crabs. Improved breed of fish, of lobsters will 

 It. The possibilities are well-nigh limitless ; and by culti- 

 )n of the sea and sea bottom, as well as of the land, man will 

 pone indefinitely the fulfilment of the Malthusian prophecy. 

 lit conditioning all advance in the possibilities of marine cul- 

 ition is the knowledge of the Plankton, of its distribution, 

 a 1 of the fundamental basis of marine life the microscopic 

 n urine organisms in the ocean. 



George W. Field. 



NO. 1200, VOL. 46] 



OPTICAL PROJECT/ON.^ 



'T'HE intention of this lecture is to give a general survey of the 

 subject of Optical Projection, which now takes its position in 

 science, and to present examples of what may be done by this 

 method. It would be difficult to determine which subject claims, 

 a first place. Some scientists .say the microscope should have the 

 preference, while others take a different view. For my own 

 part, I think the microscope and polariscope stand foremost, on 

 I account of the facility with which these branches of science may. 

 I be pursued for the benefit of a large number, without multiply- 

 : ing expensive apparatus; also because of the convenience in 

 1 saving the eyes from undue strain. Indeed, to many persons, 

 I looking at objects in the table microscope is little short of a 

 painful operation, and consequently the study of small objects 

 I becomes to them impossible. The projection method immedi- 

 ately brings the required relief. 



For general instruction, projection methods are invaluable, 



I such as, for instance, showing diagrams, photographs, and other 



slides, upon the screen ; as well as for .spectrum analysis. In 



fact, the subjects which can be illustrated by means of optical 



projection are innumerable ; but time will allow me to present 



! only a few examples, and I trust that, when I approach the end 



! of my lecture, my view of the importance of this subject will be 



held in equal estimation by you. 



Probably the only people in the world that benefit by the 

 experience of their predecessors are those who pursue the study 

 of science. They are free from the accusation of robbing the. 

 brains of other men, when they take up methods or apparatus 

 already known and improve upon them or employ them for their 

 own work. In such cases, however, it is always understood 

 that honour should be given where honour is due, and accord- 

 ingly I have no wish to represent to you any piece of apparatus 

 as of my own devising, when in reality it belongs to another. 



Few men have had a larger experience, and attained greater 

 success in optical projection, than has Mr. Lewis Wright, who 

 has embodied in his most recent forms of apparatus all that was 

 good in designs existing until his time. I have, therefore, 

 started from his models, making such modifications as I thought 

 to be desirable. Mr. Wright does not appear — if I may say so 

 — to have had much experience with the electric arc light as a , 

 radiant, and I found, at a very early stage, that great difficulties 

 had to be encountered when this light was used, chiefly because 

 the radiant approaches more nearly to what theory requires. 

 That which was easy with the lime-light became almost im- 

 possible with the arc lamp, and these difficulties had to be 

 conquered. 



Many scientific men are dissatisfied with the projection micro- 

 scope, on the ground that very high magnification does not give 

 that resolution and that sharpness which is found in the usual 

 methods of observation. This want I fully admit. At the same 

 time it is scarcely right to condemn a particular method because 

 you try to apply it to an unsuitable purpose. Hundreds of 

 thousands of subjects may be shown with the projection micro- 

 scope with far greater profit to the student than was possible in 

 the old way. The very fact that the professor can place his 

 pointer upon any part of the picture on the screen is invaluable 

 to the students. I shall, therefore, attempt to show you only 

 a series of microscopical subjects suitable for projection, and 

 shall not employ very high magnification. 



In regard to some substances very high powers may be used 

 with advantage, but much time would be lost in getting them 

 into the field and focussing them upon the screen. These, con- 

 .sequently, I omit, so that a large number of subjects may be 

 illustrated. 



It is fair to state that most of the apparatus used to-night has 

 been constructed by Messrs. Newton, of Fleet-street, and the 

 luminous pointer by Messrs. Steward, of the Strand. The arc 

 lamp is a Brockie's projector. Messrs, Baker, Watson, and 

 others have also come to my assistance. 



I will first show, on the screen, a picture of the lantern carry- 

 ing its various apparatus ; and then a few systems of lenses, 

 which may be employed for the projection microscope, as well as 

 a diagram of the microscope itself. 



Sub-stage condensers and objectives are, as a rule, made to 

 suit the table microscope. When projecting, by means of an 

 objective alone, in consequence of the screen distance being very 



' Friday eveninj? discourse delivered by Sir David Salomons at the Royal 

 Institution, on February 26. 



