ON" THE THEORY OF THE MICEOSCOPE. 233 



the number and disposition of which within a defined angular 

 space depends on the position of the mirror and the structure of 

 the preparation. 



This effect, which is not only such as might be theoretically 

 predicted, but also capable of exact calculation, may be readily 

 observed by examining the aperture images which accompany 

 the images of the object as before explained. Having placed 

 some object of the kind in question under the microscope and got 

 its detail in focus, the ocular must then be removed and the 

 image of the object in the open tube may be viewed with the 

 naked eye, or a suitably arranged microscope of weak power 

 (^^- to \o) which can be let down in the tube to the upper focal 

 plane of the objective. The image of the mirror or whatever 

 illuminating surface may be used will be seen as it is formed by 

 the undiffracted rays, and surrounded by a greater or less number 

 of secondary images in the form of impure colored spectra, whose 

 sequence of colors reckoning from the primary image, is always 

 from blue to red. Objects consisting of several systems of lines 

 which cross each other shew not only a series of diffraction 

 images of each group in the direction of their perpendicular, but 

 also, as theory would require, other additional series in the angles 

 between the perpendicular groups. Insect scales and diatom 

 valves exhibit these phenomena in the greatest variety. The 

 coarse specimens admit of examination with low powers of small 

 angular aperture ; the finer, from pleurosigma angm. upwards, 

 require large angular aperture even to get into the aperture the 

 diffraction images lying nearest to the primary image of the 

 mirror, A weak immersion lens is the best for such observa- 

 tions. 



XYI. This method of direct observation of pencils of light 

 coming direct from the object enables us, at the same time, to 

 determine by experiment what part is played by diffractive 

 phenomena in forming the image of the structure in question. A 

 suitable test object being placed in focus, and the light being 

 suitably regulated by diaphragms placed immediately alove the 



