October, 1901.] 



KNOWLEDGE. 



237 



the stores of dust. OtJier sources of atjnospheric dust 

 ai-e found in the stream of meteors which ai-e continually 

 plunging into the earth's atmosphere, their combustion 

 also resulting in atmospheric dust. Volcanoes again ai-o 

 important distributors of dust. A cigarette smoker 

 casts some 4.000.000.000 dusty atoms into the air at 

 every puff; while the shaking of door mats and other 

 similai- operations const,antly serve to launch a never 

 failing stream of dusty particles into the air. These 

 pai-t.icles of dust, it will Ix^ .seen, are the agent.s princi- 

 pally responsible for tinting the atmosphere blue and 

 for filtering out the goi-gcous hues of a sunset. 



lu respect of tlies© atoms of dust the atmosphere 

 may be likened to some brobdingnagian vessel ; for 

 these atoms ai-e always falling slowly downwards towards 

 the eai-th like particles of chalk in a glass of water. 

 As might therefore be expected, the lower strata of the 

 atmosphei-e ai-e most crowded and congested with these 

 dusty loiterers, as is well illustrated when on a calm 

 windless day these atoms settle downwards in such dense 

 crowds and multitudes as to produce a dense black fog. 

 But fai- above these lower levels the dusty atoms find 

 their way, and since they are able to float so easily 

 in these rarefied regions it is obvious that they must 

 be of a lighter build and of more attenuated proportions 

 than their relations which dwell where the air is dense. 

 Even at these great heights there ai-e ascensional 

 currents of air which keep the tiny particles of dust 

 floating. Although these particles ai'e spoken of as 

 dust, many of them are so minute that a microscope 

 fails to render them visible, and the only way in which 

 they reveal theLi' presence is by their efi'ects. Not only, 

 therefore, do dusty particles pervade the atmosphere 

 in all parts, but they vai-y in size from those that are 

 coarse and readily discernible to others that are below 

 microscopic sight. 



Dusty atoms, are fui^ther to be conceived as oflFering 

 considerable resistance to the passage of the rays of 

 light which emanate from the sun. Luminous bodies, as 

 is well known, shed rays of light of vai-ying wave length, 

 as the term is ; and as i-egards human vision only those 

 rays whose wave length is between 00036 and 00075 

 millimetres can be seen. As these waves of light sin-ge 

 through the atmosphere, not only does their wave length 

 affect their manner of passing through the earth's atmo- 

 sphere, but the different sizes of the dusty atoms against 

 which the rays of light strike introduce other modifica- 

 tions. Thus, many atoms of du.st are of a smaller 

 dimension than the wave-lengths of light that rush in 

 among them. Hence it happens that the red and 

 orange rays which are of a large wave length pass over 

 these obstacles with comparative ease ; but the blue 

 rays which are of a shorter wave length are stopped, and 

 the blue light is. as it were, txu-ned out of its course 

 and scattered. Lord Rayleigh has suggested that it 

 is to this selective scattering of the finer rays that the 

 blue of the sky is due. This action has been illustrated 

 by observing what happens when a bottle of soapy 

 water is hold up between the eye and a brilliant light. 

 Seen thus the light has a yellow or an orange colour, 

 but when the liquid is looked at sideways it appears 

 blue, the rays that have been scattered being thus made 

 visible. When looking up into the sky a similar thing 

 happens, for the blue tint is that which has been 

 scattered from the sun-beams as they splashed, as it 

 were, against the particles of dust suspended in the air. 



In the lower strata of the atmosphere the coar.scr 

 particles of dust not only scatter the waves of light, 



but they also reflect them, so that at these lower levels 

 the blue tint is diluted by white light, and is accordingly 

 not so intense as when seen, say, from the top of a 

 high mountain. At this elevation only the finer 

 varieties of dust arc floating, and there is little reflection 

 of (ho light., but much scattering, and hence it is hero 

 that, the blue attains its greatest intensity. In that part 

 of the sky nearest the sun the rays of light come in 

 a direct line to the eye of the observer, and the scattern 

 ing of tho light docs not appear so great as when ono 

 looks across tho path of the beams, and it is due to 

 this circumstance that the sky near the sun is not of 

 so intense a blue as portions of tho sky farther away. 

 A similar kind of thing happens in respect of tho clouds, 

 where dust readily accumulates, and reflecting the 

 light, produces their brilliant whiteness. At tho 

 edges of the clouds, moreover, the atoms of dust are 

 busily engaged in refracting tho beams of light, and to 

 this cause is due that brilliant fringe of brightness which 

 so often adorns many of the largest clouds. Not only, 

 therefore, does the atmosiiheric dust filter out the blue 

 light that tints the sky, but it also fabricates the pig- 

 ments that colour the clouds, effects which can most 

 rca;lily bo observed in contemplating the glories of the 

 setting sun. 



Condueted by M. I.Cro§§. 



Drawing with a Camera Lucida. — Photo-mici-ography 

 has largely displaced the use of the camera luciila for re- 

 producing structure as seen through the microscope, but in 

 numerous cases photo-micrography does not do justice nor 

 reveal details in such a manner as to permit of a proper judg- 

 ment being formed of the appearance of the subject ; photo- 

 micrography will only show one plane sharply at a time, and all 

 sense of solidity, depth, etc., is lacking. When a drawing of 

 an object is made the perspective can be reproduced and a far 

 better and truer idea given of the object generally, subject of 

 course to the dehneation being accurate, than photography will 

 permit. 



Drawing with a camera lucida is an acquirement which calls 

 for a considerable amount of practice, and is not successfully 

 undertaken without a large amount of skill in the use of the 

 pencil. This condition being fulfilled, very beautiful work can 

 be and frequently is done. 



Probably the most generally useful and popular of all the 

 camera lucidas is that known as Beale's neutral tint, in which a 

 piece of tinted glass is set at an angle of 45 degrees to the 

 eyelens of the microscope, the upper surface reflecting the 

 image to the eye. 



I liave a decided preference for this pattern, although it 

 suffers from the disadvantage of necessitating the microscope 

 being set horizontally, and the image is rcver.scd at the top and 

 bottom, while the sides remain constant. Still its simplicity 

 recommends it, and very little acquaintance with it enables one 

 to utilize all its capacity. 



For many purposes a camera lucida that works with the 

 microscope vertically, horizontally, or placed at any angle is 

 desirable, and for such the Abbe Camera is generally considered 

 the best. 



The object is drawn as seen in the microscope, and, when 

 working, the mirror reflects the image of the pencil point and 

 paper on which the pencil is tracing, into the apparent field of 

 view. 



