88 MEMORANDA. 



kind I have yet seen, I feel desirous that others should avail 

 themselves of the advantages attendant on its application to 

 the microscope. 



This condenser consists of two achromatic lenses, one of 

 four and the other of two inches focus. The four-inch lens 

 has an aperture of an inch and a quarter, the two-inch lens 

 an aperture of three-quarters of an inch ; they are placed at 

 one inch and three quarters asunder, and the compound focus 

 is an inch beyond the smaller lens. This condenser is placed 

 below the stage of the microscope, but contrived to revolve in 

 the arc of a circle, so as to vary its position from perfectly 

 direct light, to the greatest obliquity of position that may be 

 required for illuminating the most delicate linen objects. Its 

 distance from the stage when used with the higher powers 

 being such, that in every position a perfectly well-defined 

 image, either of the flame of the lamp, or the bars of the 

 distant window, is depicted on the slider holding the object. 

 The two achromatic object-glasses, which form the condenser, 

 require to be accurately made, and when so formed the light 

 Irom it is most intense and of the purest kind, at the same 

 time producing a degree of definition superior to that of any 

 other method of illumination that 1 have seen : in addition to 

 this, the illumination is equally perfect for the most oblique 

 light; so much so, that when the axis of the condenser is 

 inclined to the axis of the microscope, for the most extreme 

 angle required with lenses of 150"^ of aperture, there does not 

 appear any diminution either of light or definition. 



For microscopes furnished with this condenser no concave 

 mirror would be required ; and for illumination with the low 

 powers it is only necessary to slide the condenser further from 

 the object, so as to illuminate it by a broader part of tfee pencil 

 of light. 



The light may be either admitted directly through the con- 

 denser, or reflected through it by means of a plain mirror. 

 By the use of this condenser 1 have resolved many of the 

 delicate test-objects with a l-4th of an inch lens of 95° of 

 aperture, that would be found under ordinary illuminations 

 very difficult to resolve with a l-8th object-glass and 130^ of 

 aperture. 



The two lenses used in this condenser are constructed on 

 the same principle as all achromatic combinations for the 

 microscope, their plain sides being turned towards the object, 

 and the wider lens of course placed next the light. A good 

 workman will easily contrive an elegant method of fixing the 

 condenser to the microscope, and it may be adapted so that 

 the axis of the condenser may be brought to the rec|uired angle 



