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images became much more perfect and 

 distinct. In this way the features of 

 persons outside could be discerned, and 

 known from the image. The pictures, 

 however, thus produced, were inverted. 

 To remedy this, he proposed that, instead 

 of being projected on the wall, the rays 

 from the lens should be received upon a 

 convex mirror properly placed and 

 adapted to the lens. 



Observing that the images thus formed 

 by the convex lens were magnified, and 

 seen with more distinctness than with the 

 naked eye, it occurred to him that if a 

 convex lens were presented to an object 

 so as to form an image at its focus, an 

 eye placed a short distance behind that 

 image would see the object magnified. 

 Such an arrangement was, in fact, a 

 telescope without an eye-glass. 



(7.) The phenomenon of the rainbow 

 could not have failed to attract attention 

 at a very early period. A work has 

 been brought to light by Venturi, writ- 

 ten, in 1311, by Theodoric of Saxony, a 

 Dominican friar, in which a rational 

 explanation of the double bow is given. 

 Extensive extracts from this work, with 

 the figures of the single and double re- 

 flection within the drops, may be seen 

 in the sixth volume of the Annales de 

 Physique et de Chimie. 



The art of printing not having been 

 then invented, it is probable that the 

 work of Theodoric did not gain publi- 

 city, and was not generally known. 

 Accordingly, we find several eminent 

 philosophers, at the end of the sixteenth 

 and commencement of the seventeenth 

 centuries, engaged in attempts to solve 

 the problem of the rainbow. Some 

 conceived the exterior bow to be a 

 reflected image of the interior one, 

 and thus accounted for the inversion 

 of the colours. Fleschier, of Breslau, 

 attributed the production of the colours 

 of the rainbow to two refractions by 

 the drop, but conceived that a reflec- 

 tion took place at another drop before 

 the light reached the eye. It is curious 

 to observe how slowly and gradually the 

 laws of nature are discovered. 



Soon after the year 1600, Antonio de 

 Dominis, archbishop of Spalatro, re- 

 duced the phenomenon of the rainbow 

 to actual experiment, and proved that 

 one reflection only, with two refractions, 

 is sufficient to produce the effect. He 

 filled a hollow globe of glass with water, 

 and having placed it in a proper position 

 with respect to a beam of solar light, 

 found that when viewed in the same 



direction, and under the same circum- 

 stances, as the drops of rain which form 

 the bow, the same colours were pro- 

 duced, and were disposed in the same 

 order. He conceived that a solar ray 

 entering at the upper part of the drop, 

 was retracted to the back of it, where it 

 suffered a reflection by the inner sur- 

 face ; passing out again at the lower 

 surface, that it reached the eye of a spec- 

 tator properly placed, and produced a 

 perception of the colours of the bow. 

 He accounted for the colours in the fol- 

 lowing manner. The red rays issued 

 from the nearest part of the inner sur- 

 face of the drop, and having traversed a 

 less quantity of water, preserved the 

 greater degree of intensity ; for the red 

 colour was always considered to be pro- 

 duced by the most intense and active 

 portion of the light. The green and blue 

 rings, on the contrary, were those which 

 were reflected from that part of the 

 posterior surface of the drop which was 

 most distant from the point of final 

 emergence, and having, therefore, tra- 

 versed a greater quantity of water, were 

 more faint. The other colours of the 

 bow were conceived to be formed by these 

 three mixed in various proportions. 



If a straight line be drawn from the 

 sun to the centre of the drop, and con- 

 tinued through the centre, it will meet 

 the posterior surface, at a certain point. 

 De Dominis conceived that the rays 

 which produced the same colour were 

 similarly situate with respect to this 

 point, and therefore that such rays ought 

 to form with the line drawn from the 

 sun to the eye of the spectator equal 

 angles. Hence he inferred that each 

 colour should appear in a circular band 

 or in the surface of a cone of which the 

 eye is the vertex, and the line from the 

 eye to the sun the axis. Upon these 

 principles he accounted for the shape of 

 the bow and the order of the colours, 

 and confirmed his theory by correspond- 

 ing experiments with the glass globe. 



It has been considered extraordinary 

 that it should be reserved for De Dominis 

 to make the first important step towards 

 an explication of the most singular and 

 beautiful phenomenon in nature. Mon- 

 tucla declares his work to be obscure 

 and confused, and to betray an unusual' 

 ignorance even of so much of optical 

 science as was generally known in that 

 day. Some difference of opinion exists, 

 however, as to the extent of his claims 

 to a share in the merit of the explana- 

 tion of this phenomenon. Montucla 



