WITH THE MICROSCOPE. 249 



with much roundness of the objects, but the definition, as might 

 be expected, does not quite equal, in some of the finest markings, 

 prints obtained from sun negatives. However, all of the general 

 characteristic appearances of the objects are exceedingly perfect. 

 Great simplicity in the apparatus, and the immense advantage of 

 useful illumination in all weathers, are most favourable recommenda- 

 tions. 



332. Of the Illumination : Sunlight : Monochromatic Light : 

 Polarising Apparatus. Both sunlight and artificial light have been 

 used. Dr. Maddox, with the majority of observers, gives the pre- 

 ference to sunlight in all cases, and nearly always uses some form of 

 condenser. He usually dispenses with the mirror, and substitutes 

 one of Abraham's achromatic condensing prisms, placed at such a 

 distance from the object (if used alone) that its rays should cross 

 just before reaching it. Otherwise the intense heating power at 

 the vertex of the cone of rays would cause considerable danger to 

 the object, and might even uncement the lenses of the objective 

 of the higher powers, especially when the object is only enclosed 

 between two pieces of the thinnest covering glass, and the focus very 

 close. The prism he seldom employs alone, but places in the tube 

 at the back of the stage a condenser. A small Coddington lens 

 about 15 angular aperture, served him in the earlier part of his 

 experiments. This was made to slide nearer or farther from the 

 object. Latterly he has used Sollitt's achromatic condenser, as 

 furnishing a larger field and more free from spherical aberration. 

 This condenser, as described by the originator, consists of two 

 achromatic lenses with their plane surfaces turned towards the object, 

 and of 2 and 4 inches focus respectively, placed at the distance of 

 one and three quarters of an inch apart with a diaphragm between 

 them. The four-inch focus lens has a diameter of i^-inch, the two- 

 inch focus lens a diameter of f of an inch. Here then we have a 

 body of light, and a field beautifully illuminated when used either 

 with the plane mirror or the prism. A series of diaphragms slip into 

 the cap covering the small lens, which is turned towards the object. 

 Sometimes Dr. Maddox employs an achromatic doublet of about 22 

 aperture, or an achromatic condenser of larger angular aperture. 

 Although theoretically the angular aperture of the higher objectives 

 is narrowed by these moderate apertures, practically the intensity of 

 the illumination appears to compensate in a remarkable manner, as 

 is shown by the perfect delineation of some of the figures in the 

 frontispiece. The common plan is to use as a condenser the 

 objective next below the one used to render the photographic image ; 

 but if any form of solar condenser be employed by which the rays 



