SCIENCE OF THE SIXTEENTH CENTURY. 93 



obscura, an instrument which has in recent times become of the greatest 

 utility in practical art and scientific research. He describes how, by 

 making a small aperture in the window-shutter, the images of external 

 objects can be made to appear on the opposite wall of a dark chamber. 

 In this case the image is inverted, because the rays, entering the aper- 

 ture from the lower parts of an object, proceed in straight lines towards 

 the upper part of the wall or screen, and the rays from the upper parts r 

 also passing through the opening, cross the former so as to come below 

 them. But as the opening, however small, admits not one but a whole 

 group of rays from each external point of the object, the outlines of the 

 image are more or less blurred by overlapping of the images of the 

 adjoining parts of the object. This defect is less as the aperture is 

 smaller, but the brightness of the image is at the same time diminished. 

 The use of a convex lens fitted into a larger opening was a subsequent 

 improvement, for more light is thus admitted, while the rays cross 

 nearly at a point, namely, at the focus within the room. Porta found 

 that transparent pictures, placed in front of the lens outside, formed 

 their images on the wall of the darkened chamber. This is precisely 

 the principle of the magic lantern, and it is somewhat singular that 

 Porta did not think of trying to illuminate his pictures by artificial light. 

 This was not done until many years after by a German named Kircher y 

 who thus invented the magic lantern. Porta had much to say about 

 lenses and mirrors, and in certain passages in his book he would almost 

 appear to anticipate the actual construction of the telescope. "I 

 shall now endeavour," he says, " to show in what manner we may con- 

 trive to recognize our friends at the distance of several miles, and how 

 those of weak sight may read the most minute letters from a distance. 

 It is an invention of great utility and grounded on optical principles, 

 nor is it at all difficult of execution ; but it must be so divulged as not 

 to be understood by the vulgar, and yet be clear to the sharp-sighted." 

 The description which follows is so obscure that even the sharp-sighted 

 have been unable to make out its meaning, and every writer who has 

 quoted it has been obliged to give the passage in the original Latin. 

 If the passage have any meaning, it is a kind of reflecting telescope 

 which is indicated. But there is another passage in Porta's book in 

 which the use of a concave mirror for magnifying minute objects is 

 clearly described, and the arrangement proposed is in principle the 

 same as adopted by Newton in his reflecting telescope ; that is, the 

 image formed by the concave mirror is reflected to the eye by means 

 of a plane mirror. The objects viewed in Porta's arrangements are, in 

 this case, however, not distant objects, but minute characters, the plan 

 for magnifying them being thus described : " Place a concave mirror 

 so that the back of it may lie against your breast ; opposite to it, and 

 within the focus, place the writing ; put a plane mirror behind it that 

 may be under your eye. Then the images of the letters which are in 

 the concave mirror, and which the concave has magnified, will be re- 



