Markings of the English Gnat, &c. By Dr. Woodward. 257 



quasi beaded tissue seen in all sorts of objects examined with high- 

 power lenses. 



This brings me naturally to the observation that I think micro- 

 scopists, who are not too proud to learn, owe to Dr. Woodward a 

 debt of gratitude for the trouble he has taken, and the skill he has 

 displayed, in teaching us the precautions we ought to take in high- 

 power investigations to distinguish between the false and the true. 

 I have worked long enough at the microscope to feel that as a rule 

 one ought, like the late Lord Eldon, to " doubt " everything ; and 

 have often amused myself in producing, on well-known diatoms, a 

 series of permutations of " diftraction phenomena," and tlierefore 

 can appreciate most fully the truthful excellence of Dr. Woodward's 

 article and illustrations in the December number of the ' Micro- 

 scopical Journal.' I regard these papers on spurious appearances as 

 among the most valuable contributions to microscopical literature. 

 Dr. \^^oodward points out that detail, however clearly seen upon a 

 scale, may be more than suspected of being unreal if it seems not to 

 be confined to the limits of the scale or shell itself, but to " pass 

 off into space." A question arises as to how you are to deal with 

 the appearances, such as a fairly careful observer might get, and 

 such as I myself observed on the gnat's scale, where there was no 

 projection of the image into space ; a phenomenon with which, as I 

 stated, I am very familiar. The only suspicious point noticed 

 was an apparent alteration in the character of the detail on revo- 

 lution of the object, but the employment of light more or less 

 oblique has in most cases, and particularly in very diaphanous 

 objects, accustomed one to look at certain scales always in the 

 same position with respect to the plane of the illumination as being 

 " best seen " ; and it is to be feared that very many of us, in our 

 employment of the microscope, are apt to be led away by beauty 

 of image. 



It appears to be clear that no perfection of " condenser," and no 

 superior quality in the objective, can save us from acquiring erro- 

 neous impressions of what we see in the microscope, if we have not 

 a very distinct notion of the " pitfalls " which await us in the shape 

 of diffraction images. Dr. Woodward evidently has grasped this 

 difficult subject with a master hand ; he has used photography as a 

 witness — which to me is most satisfactory ; he has given us a most 

 valuable lesson, and I for one beg to thank him for it. 



Wasiiwood Heath, Birmingham, 

 April 3, 1876. 



