M OPTICAL PROJECTION 



7. Parts of a lantern. The diagram (fig. 8) of one of the 

 simplest forms of lantern, once general, but now generally 



confined to mere toy instruments 

 conveniently represents the es- 

 sential parts, which we will con- 

 sider singly. These are (1) the 

 light L, which may or may not 

 be supplemented by a reflecting 

 mirror M. This, as it is the 

 original source from which all 

 illumination is derived, while 

 the word * light ' may need to 

 be used in other senses, it will 

 Fia * 8 be convenient to distinguish as 



the radiant ; and as it forms no part of the lantern itself, and 

 various kinds of radiants are often employed in turn in the same 

 lantern (as when an experiment is worked out in a small way 

 with a lamp, and afterwards publicly performed by the oxy- 

 hydrogen light), we will postpone it for treatment separately, 

 and pass on here to other details. These are (2) the lantern- 

 body B, with its chimney or heat-vent. (3) The condenser c. 

 (4) The stage for slides or diagrams, s. (5) The focussing lens, 

 objective, or power, p. All improvements in lanterns relate to 

 one or other of these parts. 



8. The Body. This has two purposes : 1, to support and 

 keep in due relation the other parts ; and 2, to prevent any 

 light not utilised in the projection from scattering about the 

 room and impairing the effect. Japanned tin or sheet iron is 

 the simplest and cheapest material, and when economy is an 

 object, will really perform as well as anything else, provided 

 the optical parts of the apparatus are equally good. Thus, a 

 tin bi-unial will do all that the most expensive body can ; or 

 an experimental lantern made in this cheap way for a science 

 school, will come short in no part of the demonstration, and 

 may be within reach when a more expensive one would not 



