COKRESPONDENCE. 185 



focussing. The appearances are eminently beautiful, but of course 

 entirely illusory. The foregoing observations were made with a 

 monocular microscope, the writer not happening to have a binocular 

 at the present time. He does not apprehend, however, that it would 

 occasion any notable difference. In conclusion, some of your younger 

 readers may desire to know the method of calculating the number of 

 ocelli in any compound eye which they may happen to possess, or the 

 number contained in the field with the higher powers. The arith- 

 metical rule for finding the number of square inches in a circular 

 plane is to multiply half the diameter in inches by half the circum- 

 ference, the latter being three times the diameter, or, more precisely, 

 as 22 to 7. Thus a disk 8 inches in diameter will contain 48 square 

 inches, for 8 x 3 = 24, the half of which is 12. This multiplied by 

 4 (the semi-diameter) gives 48 square inches as the superficial area. 

 Now, by counting the number of ocelli in a straight line across the 

 field in the vertical and horizontal directions, and substituting the 

 numbers so found for the inches in the above example, we shall gain 

 the desired result. Thus, in the field given by the |-inch and ocular 

 which the writer used, there were 14^ of the ocelli in each direction, 

 which, calculated out, gives 150 in all. With the whole eye, some 

 allowance must be made if, instead of being truly circular, it is oval, 

 or irregular in outline. Ours has an average of 51 " facets" each way 

 in the plane part, besides others on the rounded edges, which gives 

 nearly 2000 optically available. 



COKRESPONDENCE. 



"Spurious Appearances in Microscopical Research." 



To the Editor of the ' Monthly Microscopical Journal.'' 



Sir, — The test Podura, with some conditions of illumination, dis- 

 plays a variety of indications of false structure, arising from the 

 peculiar form of the longitudinal ribbings. Eibs in such a direction, 

 viz. from quill to point, are characteristic of all the lepidopterous 

 scales, and the Podura forms no exception ; in this they are undu- 

 lating, and at the end of each " note " marking the rib drops, and 

 again rises with an increasing expanse. With a good object-glass 

 the continuity is unbroken, and readily seen and traced. I am not 

 inclined to reopen the discussion at present, but take my stand for 

 this positive assertion from having examined torn specimens with 

 ribs partly isolated or displaced, so as to give a clear idea of their 

 form ; I have collected a number of these with the view, if requisite, 

 of proving the structure by photographs of fragments.* 



* Besides examples of longitudinal and cross fractures, I have a fine slide of 

 test Podura mounted by Richard Beck, kindly presented to me by Dr. Gray. 

 The cover has evidently shifted or spun round while pressing on a large scale, 

 tearing the membrane in the middle of the specimen, and taking with it the 

 fractured ribs at the spot. Some of the club markings are twisted transversely, 

 one so far that the tbick end lays the reverse way, as clearly and definitely as a 

 half turn given to the handle of a copying-press. 



