THE BOOK OF THE LANTERN. 241 



powers of the instrument are employed, is far from being 

 enough. The electric light would obviously be the best 

 form of illuminant to use for the microscope, but the 

 incandescent variety is far too feeble, and the arc form 

 possesses neither the steadiness nor the accuracy and per- 

 manence of centreing which is so requisite in microscopic 

 work. So to the limelight we must at present confine our 

 attention, aiding it as far as possible by careful arrange- 

 ment of the lenses used in conjunction with it. Hence the 

 condenser must be of the best form, and must be seconded 

 by a substage condenser suited to the objective or power 

 which happens to be used. Provision must also be made 

 to filter the light through a layer of alum solution, which 

 absorbs the heat rays, and saves valuable objects from 

 being destroyed. 



Having secured the brightest light possible, and done 

 our best to concentrate it upon the object, we have 

 next to consider the best means of forming an image of 

 that object upon the screen. An objective may do excellent 

 work with the ordinary microscope, bat utterly fails to 

 give a satisfactory image on the lantern screen. Perhaps the 

 definition in the centre of the disc is satisfactory, but the 

 margins are all hazy and indistinct. One may examine a 

 large number of objectives with the lantern microscope before 

 one is found suited to the work. But makers are now 

 giving serious attention to the requirements of the recently 

 perfected instrument, and objectives of great excellence 

 can be obtained. 



One of the most perfect as well as simple arrangements 

 for showing microscopic objects with the ordinary lantern 



