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", and i"" T"', at the other side of 
the lens. Thus the image of the straight or fiat object o' o' is the 
curved image T T, concave towards the lens L L. In like manner, 
11, concave towards u, is the image of the object o o, which is 
convex towards il; i" i", concave towards ll is the image of 
o" o", also concave towards l l ; while the fiat image T" i'" is that 
of the object o’" o'", 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' were projected upon a screen held at the point 
where its curved image i' T 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 
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