95 



NOTES. 



A NEW LOW-POWER CONDENSER 



By Edward M. Nelson, F.R.M.S. 



{Read November 26th, 1912.) 



The condensers which at present are supplied with microscopes 

 are only suitable for low powers ranging from | inch upwards. 

 With powers lower than these a difficulty arises, for it is not 

 possible to fill the field with the image of the source of light 

 focused upon the object, as it should be. Substage condensers 

 suitable for low powers are all too short in focus, consequently 

 the image of the source of light is far too small. 



In these circumstances microscopists have been, and are, 

 accustomed to waive critical illumination and employ the most 

 uncritical of all illumination, viz. to focus the image of the 

 source of light upon the front lens of the objective ; this is 

 nothing more nor less than lantern illumination, which gives 

 a critical image of a diaphragm limiting the field, but of nothing 

 else ; all delicate lines and structures are coated with black 

 diffraction borders. 



The obstacle in the way of using a long-focus condenser is that 

 there is not sufficient room to focus it. 



Powell's No. 1 stand has a good deal of room, but not enough, 

 and other microscopes are simply nowhere. Now the way this 

 difficulty may be surmounted is to construct the condenser upon 

 the telephoto principle. This has now been done, and Messrs. 

 Baker will show you this evening a substage condenser they have 

 made from my design which has 4 inches of focal length and 

 requires only 1 inch of working distance. With this condenser 

 the image of the fiat of the flame bears the same relation to a 

 4-inch objective with the large field of a P. & L. No. 1 A eye- 

 piece, as the image with one of the ordinary universal condensers, 

 with the top off, does to a | inch ; and this is precisely what 

 was wanted. 



Now let us understand exactly what this means. A 4-inch 

 objective has a focal length of 2| inches; with a No. 1 A 

 eyepiece the size of the object on the stage that is embraced 



