6 OPTICAL PROJECTION 



some fixed distance related to the other distance, according to 

 the refractive power of the glass. It is accordingly called the 

 conjugate focus of the point/. 



Here we have exactly what we wanted to get a brighter 

 image. To make it more clear, take both the slide and the 

 condensers also out of the lantern, leaving only the perforated 

 tinfoil with its five holes, and let the object be the light in 

 the lantern itself, because its images will be both brighter, and 

 will stand out obviously apart upon the screen. Now take a 

 lens ! in the hand, and hold it close to the tinfoil. At 

 once the outside images will be bent in towards the centre 

 one, and a position will be readily found in which all are 

 exactly made to fall on one spot. Finding that position, and 

 placing the lens there, is ' focussing ' the lens. This image 

 is obviously five times as bright as the original one, and if 

 now the lens can be supported in that position, we may prick 

 any number of holes, and break away the tinfoil altogether, 

 and it makes no difference except to further brighten the image ; 

 because in the same way all the images formed by every ray 

 are bent in to the same point, and unite their brightness .in 

 one. 



This is the only action of a projecting lens. People talk, 

 and even write, as if it formed the image, or inverted the 

 image ; but it does nothing of the kind. The rays themselves 

 form the images ; and their crossing at the place where the 

 lens is, inverts the image ; and all the lens does is to bend 

 into one spot the rays (forming images) which fall upon its 

 surface, and so to combine countless faint and jumbled 

 images into one bright and clear one. That is the true 

 nature of Optical Projection. 



This much being experimentally realised, several very 

 important principles at once become clear. First of all, it 



1 This lens must be of somewhat longer focus than the distance from the 

 light to the tinfoil, or the light must be pushed up to the front so as to malca 

 ihat distance less than the lens focus. 



