ZOOLOGY AND BOTANY, MICKOSCOPYj ETC. 529 



few insect scales. These are brought into focus by a low-power 

 objective used as a focusing-glass, and the image of the object on the 

 stage of the Microscope and the image of the scales are made to coin- 

 cide. Hence, by withdrawing the little camera and inserting the 

 focusing objective, the focus of any object on the stage can be made to 

 occupy the exact position of the scales on the transparent glass. In 

 other words, the focus of these and the new image are coincident, 

 and, the surface of the plate falling exactly in the same plane, there 

 can be no error through the diflferent thickness of the glass-plate. 



The miniature camera of Messrs. Robinson is well suited for taking 

 such negatives as those recommended by Dr. Boux, It is only 3 in. 

 by 21 in. 



Dr. H. Van Heurck* also describes a very small mahogany 

 camera, extremely light, receiving at its posterior part a gelatino- 

 bromide plate of 4^ cm. by 5^ cm. Anteriorly the camera carries a 

 copper tube 5^ cm. in length, terminated by a Zeiss amplifier. The 

 copper tube enters the tube of the Microscope a short distance. 



Amphipleura pellucida and the Diffraction Theory. — The 

 photographs of this diatom recently made by Dr. Van Heurck have 

 given rise to some discussion, and some of those who do not admit the 

 reality of the beaded appearance shown by the photographs, claim to 

 rest their view on the Abbe diffraction theory. 



This shows that some misconception exists as to the application 

 of the theory, which does not establish, as supposed, that all appear- 

 ances of minute structure with high powers are wholly illusory and do 

 not correspond to any physical structure. On the contrary, the images 

 shown by the Microscope are all, in fact, caused by real structural 

 peculiarities of the object observed. Thus in the case of the " beads " 

 of A. pellucida, the existence of such an image proves that the diatom 

 has not merely a periodic differentiation of structure in one direction, 

 but that such differentiation exists in two directions which cross at 

 right angles. 



What the diffraction theory shows is that the real form and struc- 

 ture of the beads cannot be determined by the mere inspection through 

 the Microscope of their images. The Microscope leaves wholly un- 

 decided the question whether they are elevations, or depressions, or 

 simple centres of thickening in the substance of the valve, resulting, 

 it may be, from the intersection of two siliceous layers, the densities 

 of which vary periodically. 



* Amer. Mon. Micr. Journ., vi. (1885) pp. 42-5. 



Ser. 2.— Vol. V. 2 M 



