538 



NATURE 



{Oct. 7, 1880 



thrown by the lens in front of it. It is possible indeed 

 to show in actual fact that the image in the eyeball is 

 inverted ; the experiment is very simple, but we believe 

 that this is the first time that it has been described in 

 print. Take a candle, and hold it in your right hand as 

 you stand opposite a looking-glass. Turn your head 

 slightly to the left while you look at the image of yourself 

 in the glass. Open your eyes very wide and look care- 

 fully at the image of your left eye. Move the candle about 

 gently, up, down, forward, &c., so that the light falls more 

 or less obliquely on to the eyeball. You will presently 

 notice a little patch of light in the extreme outer corner of 

 the eye : it is the image of the candle on the inside 

 of the eyeball, which you see through the semi-trans- 

 parent horny substance of the eye. If you move the 

 candle up, the little image moves down, and if you suc- 

 ceed well, you will discern that it is an inverted image, 

 the tip of the flame being downwards. You thus prove 

 to your own satisfaction that the image of the candle in 

 your eyeball is really upside down. 



Fig. 23 shows a magnifying-glass of very simple con- 



struction, which a few years ago found a great sale in the 

 streets of London, at the price of one penny. A bulb 

 blown at the end of a short glass tube is filled with water. 

 When held in front of the eye, this form a capital lens 

 for examining objects of microscopic dimensions, which 

 may be secured in place by a bit of wire twisted round 

 the stem. 



The principle by which the intensity of two lights 

 is compared in the photometer is very easily shown. 

 The arrangement depicted in Fig. 24 shows how to 

 measure the relative brightness of an Argand oil-lamp 

 and of an ordinary candle. Both these lights are 

 set upon the table, and are so arranged that each 

 casts on to a screen of white paper a shadow of a 

 tall narrow object. The most handy object for this pur- 

 pose is another candle unlighted. The Argand lamp, 

 being the brighter light, will cast the deeper shadow of 

 the two, unless it is placed further away. The measure 

 of the brightness is obtained by moving the brighter light 

 just so far off that the intensity of the two shadows is 

 equal, for then we know that the relative intensities of the 



two lights are proportional to the squares of their distances 

 from the photometer. All that remains, therefore, is to 

 measure the distances and calculate out the intensities. 

 If, for example, the distance of the lamp is double that of 

 the candle when the two shadows are equally dark, we 

 kno.v that the brightness of the lamp is four times as great 

 as that of the candle. 



Many other facts in optics can be shown with no greater 

 trouble than that entailed by such simple experiments as 

 we have described. The pendant lustre of a chandelier 

 will provide an excellent prism of glass for showing 

 the dispersion of light into its component tints. A 

 couple of spectacle glasses appropriately chosen will, 

 when pressed together, afford capital " Newton's rings" 

 at the point where they touch. Diffraction bands of 

 gorgeous hue may be observed by looking at a distant 

 gas-hght, or at the point of light reflected by a silvered 

 bead in sunshine, through a piece of fine gauze, or 

 through a sparrow's feather held close in front ot the 

 eye. And yet more remarkable effects of diffraction 

 are obtained if the point of light be looked at through 

 substances of still finer structure, such as the preparations 



of woody structure, and of the eyes of insects which are 

 sold as microscopic objects. But the explanation of 

 these beautiful,' phenomena would lead us far beyond our 

 subject. 



( To be continued.) 



THE JAMAICA HURRICANE AND THE 

 BOTANICAL GARDENS 



T 



HE following letter from Mr. Morris has been for- 

 warded to us from Kew for publication : — 



Botanical Department, Gordon Town, 

 Jamaica, Scptendter"], 18S0 

 At the Cinchona Plantation, besides damage to our 

 buildings and sheds of about 650/., our nurseries and seed 

 beds have suffered so much as to reduce our stock of 

 available seedlings from something lik* 500,000 down to 

 So,ooo. These were intended for planting out in the 

 latter part of this year and the beginning of the next year. 

 We shall in consequence be unable to distribute seedhngs 

 as we intended, and so suffer considerably in expected 



