THE PROJECTION MICROSCOPE 193 



alteration in the curves, such a lens, on the Petzval principle, 

 does all that can be desired. For the common range of work 

 with the lime-light, where results with lenses already possessed 

 are not satisfactory, the following series is recommended, and 

 will be found perfectly so : 



Focus Diameter covered Price 



a. d. 



2| inch (Petzval) \\ inch 12 6 



ll inch f inch 250 



j'oinch 1 inch 276 



inch T \ inch 1 10 



The first three of this series being worked out for projection 

 direct to the screen, do not behave so well with an amplifier 

 for anything critical, though one may be used ; but for direct 

 projection, though moderation of price has been studied in 

 their construction, and especially since their formulas have 

 been further perfected by the use of the new Jena glass, the 

 1 J inch and T 8 inch are the finest lenses yet produced, being 

 far superior in illumination, in definition, and extent and flat- 

 ness of field, to all others constructed for the same purpose 

 which I have been able to test, including both American and 

 Continental lantern objectives of a very much higher price. 

 The Ti5- works equally well with or without amplifier, or with 

 an eye-piece, and may be pushed up to 1,500 diameters, which 

 means a blow-fly's proboscis 14 feet long. 



Working with a proper projecting eye-piece, a much larger 

 number of ordinary micro-objectives will do good work, only 

 the best third of the field being used, and that much flattened 

 by the field-lens. Almost any really fine lens will do fair 

 work in this way. I mention the following as lenses I have 

 actually tested. Zeiss's aa (1-inch) gives magnificent defini- 

 tion in the centre of the field, both direct and with an eye- 

 piece, but is far from flat. Powell & Lealand's J inch of 40 

 is very crisp, and works well either direct, or with amplifier 



1 I am endeavouring to get this lens modified into an inch, which with the 

 Reichert f presently mentioned, will give a more useful gradation of powers. 







