Oct. 3, 1884.] 



• KNOW1.EDGE • 



271 



ture, its dipressions and borders, separating one from 

 another, are as plain as the cells of a honeycomb, but if 

 viewed at night, and strongly illuminated from one side, it 

 is very likely to look as if every depression was an eleva- 

 tion. Sometimes if this effect is produced with one-eyed 

 vision, a right view can be obtained by a binocular one, 

 but sometimes not. There are many cases in which 

 depressions look like elevations. Examining the moon 

 through a telescope leads some folks, not inft-equently, to 

 see all the crater hollows as projecting mounds, and when 

 the eye has once been deceived, the false view is apt to be 

 excessively troublesome. Nasmyth's admirable moon 

 slides for the lantern frequently puzzle observers, and 

 their difficulty in seeing the hollows lower than their elevated 

 surroundings is increased if slides with shadows to the 

 right succeed others with shadows to the left. Hollow 

 casts of gems in plaster of Paris, or sulphur, can easily be 

 made to look like cameos, either by help of a magnifying 

 glass or a simple hole, a quarter of an inch in diameter, 

 cut through a blackened card. The experimenter with the 

 thimble can usually arrange the light, itc, as to show the 

 true structure, but if the eye has once caught a wrong 

 picture, it may refuse to have it corrected. Besides looking 

 at the thimble itself, impressions in black and red sealing- 

 wax should be examined. 



To take another instance : a transparent film, such as one 

 of colloid silica, which cracks in drying on a glass slide, 

 may be (mployed. Very clear gum or thin white varnish 

 will perhaps do as well. The cracks arc easily recognised 

 by holding the slide against the light. One, perhaps, begins 

 with a pretty wide fissure, and thins out to a very narrow 

 one. The narrow part will seem, when magnified one or 

 two hundred times linear, to be elevated like a slender 

 rod superposed upon the slide. This will show how 

 cautious the observer should be when studying many 

 classes of objects. 



Similar to the disputes about diatom structure are those 

 about scales of butterflies and moths. The questions are 

 how many membranes go to form a scale. Is there one 

 under and one upper, or more ? Are the ribs between the 

 membranes or growths in their structure 1 Are the minute 

 markings composed of dots of pigments, or of other shapes? 

 In what plane or planes do the several formations lie 1 ifcc. 

 When high powers are employed, and it is desired to ascer- 

 tain in a very thin object what structure is on the surface 

 and what others in successive planes below, the fine adjust- 

 ment usually fitted to a microscope is not nearly so delicate 

 as it is advisable to have it. Dr. Pigott in such cases 

 adapts a second wheel, moving with a wonderfully fine 

 screw of his own cutting, to the one sent out with his 

 Powell and Lealand microscope. He can show that with a 

 l-16th or a l-2.3th objective and a delicate object, a change 

 of 100-lOOOths of an inch in the focus produces a notice- 

 able alteration in its appearance. 



Mr. Washington Teasdale devised an excellent test-slide, 

 helping us in these matter?. One he kindly ruled for the 

 writer consists of ten groups of parallel lines, at the rate 

 of 2,000 per inch, disposed in a symmetric pattern of 

 eccentric radials. The lines are ruled with a conchoidal 

 ftacture, and not a clear cut, so as to give a maximum of 

 brilliance. This slide is to be illuminated by a spot-lens, or 

 dark ground arrangement of condenser. If the light is 

 exactly centred, all the bands are made to glow with equal 

 lustre. Any slant of the light beams brings out those 

 more plainly which lie at right angles to its Hue of inci- 

 dence. Instead of spot-lens, or condenser, obtain a dark 

 ground illumination from the sub-stage mirror turned 

 strongly on one side, and then rotate the object and note 

 the changes that ensue. 



The observer will soon find that with one illumination 

 certain bands will appear as if on a ))lane above the other.s, 

 but by a change in tlie angle t f illumination, can be made 

 to look in one below them. 



Most of your readers will have seen compound vibration 

 patterns ruled with two pendulums, or with more com- 

 plicated apparatus. When done on glass, on a smaller scale 

 for viewing under the microscope, they are instructive 

 as well as beautiful. Having been amply supplied with 

 admirable specimens by Mr. Teasdale, the writer has been 

 able to exhibit the wonderful perspective effects to be 

 obtained by varied modes of illuminating them. Trial 

 slides and damaged ones are very instructive, as any 

 rough edges come out with so much force as to mislead the 

 eye about their relative position. 



The figure — one of Mr. Teasdale's — appended hereto, 

 shows a pattern which is very deceptive under the 

 microscope. It is impossil)le to avoid the impression 

 that the observer is looking at a hollow object 

 Perspective effects, without shading, are perhaps more 

 striking to persons who are accustomed to watch them out 

 of doors, and also as exhibited in outline etchings A very 

 slight guidance induces the eye to accept the idea the 

 artist wishes to convey, and when similar guide-lines occur 

 in a piece of mechanical ruling, their intimation is imme- 

 diately followed. The subject is a very interesting one, 

 partly optical and partly psychological. To the micro- 

 scopist its study is of great importance if errors of inter- 

 pretation are to be avoided. When the optical aspect is 

 of doubtful trustworthiness, resort should be had to reason- 

 ing from analogy. 



For example, the structure to be made out in the 

 coarsest butterfly scales may help to guide to what is pro- 

 bable in others more difficult, and with delicate organs of 

 insects, rotifers, &c., it is prudent to doubt any appear- 

 ance that seems to contradict the general rule of such 

 organisations. This doubt, however, must not be carried 

 too far, and the happy mean can only be reached as the 

 result of long study and experience. 



The largest organ in the world lias just been completed by 

 Walck, of Ludswigburgh, and placed in the cathedral Church of 

 Eiga. The colossal instrument measures 3Gft. in width, 32 ft 

 from back to front, and 65 ft. high. It contains no less than 6,826 

 pipes, distributed among l'2\ sounding stops. 



