28 THE BOOK OF THE LANTERN. 



give a flat field, and the ordinary diameter of the back lens 

 of such a combination will not admit the whole of the 

 rays from the condenser : hence we are robbed of light, 

 and very often of the corners of square pictures into the 

 bargain. 



Of late years makers have seen these disadvantages, and 

 have produced lantern objectives which, while they are 

 similar in construction to portrait objectives, are made with 

 much larger apertures at the back. Taking Mr. Dallmeyer's 

 lantern objective as the prototype of these, we find that it 

 consists, like the portrait lens, of two combinations. The 

 back one, next the light, is a convexo-concave of flint, and 

 another of crown glass, separated by a short interval, the 

 two glasses being dissimilar in their curvature. The 

 external form of this combination is a meniscus, its convex 

 surface being next the condenser. The front combination, 

 of smaller diameter, also has the external meniscus form, 

 but consists, like its fellow, of two glasses. 



It must be noted that objectives made for lantern work 

 are not suitable for photography, for the visual and chemical 

 rays are not coincident. I mention this for the sake of 

 those who wish to use the lantern as an aid to their photo- 

 graphic pursuits, in enlarging and so on. It may^theref ore, 

 be in some cases desirable for the purchaser to obtain a 

 half -plate portrait lens for his lantern. It will do excellent 

 work, with the limitation already referred to, while at the 

 same time it can be used either for portraiture or for 

 enlarging. For both these uses it is well adapted. 



The rule for calculating the distance of the lantern from 

 the screen in order to obtain an image of a given size will 



