August 1, 1898. 



KNOWLEDGE 



189 



in the middle of the field, the centreing may be regarded 

 as correct. Upon the next move depends the success of 

 the experiment. Eack down the substage condenser until 

 its front is about half an inch from the object, and then 

 proceed very slowly, by means of the coarse adjustment, 

 to rack the body tuba of the microscope back from the 

 object. The hexagons will go out of focus, and nothing 

 can be seen. Continue the backward movement a little 

 farther, and a number of points of light will appear, 

 disposed in rows across the field. Focus with the fine 

 adjustment until sharply defined, and it will be seen these 

 points of light are inverted images of the lamp-llame, each 

 one formed by a "lens" iu the cornea. By racking the 

 Abbe a Uttle nearer to the stage the image in each 

 facette will be seen to increase in size, and rice tvrsd. 

 The focus of the objective remains fairly constant for all 

 positions of the condenser, provided this latter is not too 

 near. Tracing the path of light (Fig. 2) we find an image 



<r£>^^ixi\ 



C'Afi 



C3 



Fig. 2. — The " Kye-lenses," B, act as Telescopes 

 Aerial foci at A and C. 



of the radiant is formed in the principal focus of the con- 

 denser, at A, which, being at a considerable distance from 

 the cornea, is practically in the indefinite (anterior) focus 

 of the " eye-lenses," B. An image is again formed in the 

 principal (posterior) focus of each '• eye-lens," at C, and 

 when this plane C is in the focus of the objective the observer 

 at the microscope can see an image of the radiant through 

 each facette in the field. The corneal convexities, however, 

 are not all of the same diameter and radius, so that where 

 a number will yield a sharp outline of the radiant, others 

 will produce a blurred and indistinct image ; but as the 

 area covered by a half-inch objective is very small, there 

 should be little difficulty in finding a group of facettea of 

 similar focus. The subject to be photographed may now 

 be considered ; it should be either self-luminous or white. 

 If the former, a window with open landscape beyond will 

 answer, but on account of its large size must be relatively 

 more distant from the instrument. By artificial light, 

 opaque figures pasted on a ground-glass screen illuminated 

 from behind may be tried, but the most satisfactory 

 photographs are those of white subjects by reflected light, 

 such as a bust or statuette. For the purpose of my 

 experiment I have chosen a small chalk bust of Her Majesty 

 the Queen, and is it not appropriate that the noblest and 

 greatest monarch the world has ever seen should be the 

 subject of a photograph through the most infinitesimal lens 

 known to science? Remove the lamp, and in its stead 

 place the object to be photographed in the optic axis of the 

 apparatus, and, say, eighteen inches distant from the con- 

 denser ; the exact distance will depend upon the amount of 

 subject it is intended to include. It will now be necessary to 

 bring to bear upon the subject all the available illuminating 

 power at our disposal. Two Welsback gaslights, being 

 easily obtained, will do. Place one on either side of the 

 subject in such a manner that their combined rays shall play 

 upon the surface to be photographed. Two curved pieces of 

 new tinplate, placed between the burners and the camera, will 

 improve the illumination and prevent any direct light from 

 entering the condenser. Both burners and reflectors should 

 be as near the object as possible, but must not trespass upon 

 the subjective field. Examine through the microscope, 



using the substage pinion freely, until an image of suitable 

 size and definition is seen in each facette. Attach the 

 camera and fine focussing rod and see that all is Ught- 

 tight ; the bellows may be stretched twelve to eighteen 

 inches. Iq spite of the large amount of light reft acted 

 from the subject the rays transmitted by the facettes 

 will be extremely famt, owing to their minuteness. 

 They rarely exceed one-thousandth part of an inch in 

 diameter, and it will be found impossible to project 

 through them an image visible upon the ground-glass 

 screen, though the latter be most finely obscured. But 

 the photomicrographer will have more refined methods 

 at his disposal to meet the greater delicacy of his 

 work. Having removed the obscured screen from the 

 frame, insert in its place a piece of plain glass of the same 

 size ; a spoiled negative from which the film has been 

 stripped will answer excellently. A focussing eyeglass 

 will now be required, and it must be so adjusted that when 

 resting upon the plain glass screen the furthermost surface 

 of the latter is in focus ; this is best accomplished by 

 applying with the finger scales from a moth's wing, or 

 other minute particles upon that surface. Having arranged 

 the eyeglass to our satisfaction, we can return the plain 

 screen to the camera and proceed to locus the subject. 

 To do so the operator must retire a little fi-om the eyeglass 

 so that its lens is seen to be full of light ; this wiU occur 

 when his eye and the screen are equidistant from the lens 

 and in its principal focus. The writer uses a glass in 

 which both distances are determined by a tube. But the 

 images which appeared to the observer at the eyepiece of 

 the microscope will not be in focus at the screen. We have 

 lengthened the major conjugate focus of our apparatus and 

 must therefore shorten the minor. By means of the fine 

 adjustment rod, cause the objective to approach the object, 

 keeping a sharp look-out for images through the focussing 

 glass. It is not by any means easy to determine the exact 

 point at which the images are best defined, and probably 

 many " ins and outs " will be tried before a satisfactory 

 focus is established. Presuming the objective to be of the 

 usual achromatic type corrected for an optical focus, an iso- 

 chromatic plate — the most rapid obtainable — should be 

 employed, as the plates so designated are extra sensitive to 

 the yellow or visual rays within the C and E lines of the 

 spectrum. Exposure will depend, among other things, 

 upon the desired size of the images and consequent camera 

 stretch, and may be as much as, or more than, three hours. 

 I shall not here enter into details of development ; that 

 formula with which the operator has had most experience 

 is the best. Although the first plate nunj prove a success, 

 it is advisable not to remove any of the apparatus until a 

 satisfactory negative has been obtained. 



In the distribution of visual organs nature has been 

 most lavish to the insects, and we are filled with astonish- 

 ment when we reflect that from a dragon-fly's head we 

 could obtain twenty-five thousand perfect lenses, so minute 

 that a million of them would not cover a square inch of 

 surface, and yet each be capable of yielding a recognizable 

 photograph. Had nature provided man with eyes in 

 simDar profusion, how much more could he have seen of 

 her wisdom ! 



'■ I. like Samson, would have eyes at every pore. 

 To see the light and learn of truth the more." 



NOTES ON COMETS AND METEORS. 



By "W. F. Denning, f.k.a.s. 



The Discovery of Comets. — In the last number attention 

 was drawn to the relative scarcity of new comets discovered 



