The Lantern Mieroseope. By Lewis Wright. 197 



material size, we have necessarily a very much larger image of the 

 radiant instead of a point. The small movable lens will no longer 

 collect more than a small part of the hght ; and taking the Hme- 

 light even at 500 standard candles, we have no light to spare. 

 Hence the failure of all instruments modelled on the usual theo- 

 retical diagrams, or hehostat practice. 



This is not quite all. It is plain that we must condense, as 

 nearly as possible, all the light upon the object, however small its 

 size. And it is further plain, that the smallness of this illuminated 

 spot must be a function of the relative foci of the large lantern 

 condenser, and the final secondary condenser. But if we use for 

 the latter a large lens of very short focus, then we get such a high 

 angle that most of the light crossing in and diverging from the 

 object, never passes through the objective. If on the other hand 

 we use a very smaE lens, most of the light never gets through this 

 lens. 



Hence the matter becomes one of practical adjustment, at every 

 point ; and each secondary, or as we may perhaps call it, substage 

 condenser, must be specially constructed for powers of a certain 

 range only, in focus and angle. Except for low powers, each sub- 

 stage condenser is almost necessarily composed of two lenses ; one 

 of fairly large size, to take up and condense or bring down all the 

 cone of light ; the other, to still further condense that light to the 

 size of the object desired, without employing an angle too great for 

 the objective. There was further to be studied perfect protection 

 from the heat, which is very great ; and absolute simplicity of parts 

 and manipulation, without which work in the dark cannot be effec- 

 tively performed. In regard to several of these objects, I felt it very 

 desirable that such an instrument should be constructed by opticians 

 specially familiar with lime-light projecting apjDaratus, and who 

 would test every instrument actually upon the screen. For want 

 of this latter precaution, one lantern Microscope lent me behaved 

 far worse than another of the same make : copied unintelligently, 

 and sent out without testing (for it never could have been tested), 

 it was really unworkable, and no one had ever been able to do 

 anything with it. It was also natural to expect that such opticians 

 would more readily understand the reasons for some of my arrange- 

 ment, and sympathize with my passion for simjplicity, without 

 contriving or urging upon me extra provisions of screws and other 

 brasswork, so dear to the average instrument maker, and proper 

 enough for many purposes, but which in the dark would be to a 

 practical demonstrator a source of unmitigated distress. 



All these expectations were realized, the instrument being 

 completed by Messrs. Newton and Co., with both care and skiU, 

 It is shown in fig. 40. Some points in it they have, in the exercise 

 of an equitable right I was bound to recognize, desired to protect 



