18 THE MIOKOSCOPE. 



mass of information of this sort, and to these we refer the 

 reader. 



The mode of illuminating objects is one on which we must 

 give some further information, for the manner in which an ob- 

 ject is lighted is second in importance only to the excellence 

 of the glass through which it is seen. In investigating any 

 new or unknown specimen, it should be viewed in turns by 

 every description of light, direct and oblique, as a transparent 

 object and as an opaque object, with strong and with faint 

 light, with large angular pencils and with small angular pencils 

 thrown in all possible directions. Every change will probably 

 develop some new fact in reference to the structure of the ob- 

 ject, which should itself be varied in the mode of mounting in 

 every possible way. It should be seen both wet and dry, and 

 immersed in fluids of various qualities and densities, such as 

 water, alcohol, oil, and Canada balsam, for instance, which last 

 has a refractive power nearly equal to that of glass. If the 

 object be delicate vegetable tissue, it will be in some respects 

 rendered more visible by gentle heating or scorching by a clear 

 fire placed between two plates of glass. In this way the spiral 

 vessels of asparagus and other similar vegetables may be beau- 

 tifully displayed. Dyeing the objects in tincture of iodine will 

 in some cases answer this purpose better. 



Bat the principal question in regard to illumination is the 

 magnitude of the illuminating pencil, particularly in reference 

 to transparent objects. Generally speaking the illuminating 

 pencil should be as large as can be received by the lens, and no 

 larger. Any light beyond this produces indistinctness and 

 glare. The superfluous light from the mirror may be cut off 

 by a screen having various-sized apertures placed below the 

 stage; but the best mode of illumination is that proposed by 

 Dr. Wollaston, and called the Wollaston condenser. A tube is 

 placed below the stage of the instrument containing a lens A B 

 (Fig. 11), which can be elevated or depressed within certain 

 limits at pleasure; and at the lower end is a stop with a limited 

 aperture CD. A plane mirror E F receives the rays of light 

 L L from the sky or a white cloud, which last is the best source 

 of light, and reflects them upwards through the aperture in 

 C D, so that they are refracted, and form an image of the 



