OPTICAL IMAGES. 



The relative forms of the object and its image in such case will 

 be more clearly understood by reference to fig. 28, where L L is 



Fig. 28. 



the lens, and o o, o' o', o"o", o'" o'", and o"" o"", objects having 

 the different forms above mentioned, placed at a point beyond its 

 principal focus. The images of these severally are indicated by 

 the letters 1 1, i' I', i" i", i"'!'", and i"" i"", at the other side of 

 the lens. Thus the image of the straight or flat object o' o' is the 

 curved image i' I', concave towards the lens L L. In like manner, 

 1 1, concave towards L L, is the image of the object o o, which is 

 convex towards L L ; i" i", concave towards L L is the image of 

 o" o", also concave towards L L ; while the flat image i'" i'" is that 

 of the object o'" o' r/ , which is curved and concave towards L L. 

 The image i"" i"", convex towards L L, is that of o"" o"", concave 

 towards L L. 



It will be evident that none of these images could be projected 

 with uniform distinctness upon a flat screen, except that of the 

 curved object o'" o'", the image of which is flat. If the image of 

 a flat object o' o 7 were projected upon a screen held at the point 

 where its curved image i' i' intersects the axis of the lens, it 

 would only be distinct at and near the centre. The screen being 

 behind the extremities would be out of focus with them, and 

 consequently those parts of the image would be indistinct. If the 

 screen were advanced, so as to render the extremities distinct, the 

 centre would be out of focus, and consequently indistinct. 



In this case, the object is assumed to be placed beyond the 

 focus of the lens, and consequently the image is always real, 

 whatever be its form. Let us now consider the case in which 

 the object is placed within the focus, and its image consequently 

 imaginary (34). 



Let L L, fig. 29, be the lens, and let the object, placed within its 

 106 



