Minute Organic Particles. 23 



These were the shadows, the mixed crescentic shadows of the 

 now recognized beading. 



So also, Dr. Goring's beautiful drawings of lines, and cross-lines 

 as seen on the Podura, really predicted the beaded structure 

 developing those mixed shadows. It is still easy with inferior 

 glasses to get as many as three sets of lines in many beaded objects 

 as that in the Angulata. I have shown the crossing striae of the 

 Podura to many persons. The cords described in the note are, 

 perhaps, the most interesting part of his research.* 



V. The nature and characteristics of perfect definition. 



If ever you are about to buy an objective, try it with the 

 deepest eye-pieces you can find ; and after making every adjustment 

 practicable to correct the aberration of the eye-pieces (for they all 

 difier in this respect), observe whether the image becomes woolly, 

 indistinct, blurred, hazy ; regard the edges, and examine an object 

 with obHque as well as direct light. You shall see all these dis- 

 comforts increase as the eye-pieces are deeper still. The glass will 

 not bear such scrutiny unless of very fine quality. Do not pull 

 out the tube : that destroys the previous correction. Get all the 

 definition you can by using another fine objective for a condenser. 

 Try it with the mirror, also by oblique sunlight. The better the 

 glass the less will it be dependent upon tricks of illumination, stops, 

 condensers, and obliquity. I have a glass which performs almost 

 as well with the usual mirror as with the most perfect condenser. 

 You constantly hear people say, " Oh, that is too deep an eye-piece." 

 Of course ; it is too candid. Now, what really are the obstacles to 

 splendid definition ? I fearlessly say that the chief of them may be 

 summed up in two or three words. 



Workmanship ; displaced foci. 



The workmanship is often the worst feature. Each uncentred 

 glass gives out its own displacement of the circle of least confusion. 

 I very rarely find the centres perfectly coincident. This blurs 

 the image. 



Displaced foci are always present. 



Perfect definition requires that all the coloured rays, supposing 



* Dr. Goring, who may justly be described as the inventor of test-objects, says 

 (' Mic. Cabinet,' p. 152) : — " The Podurse show a series of lines or cords on their srtr- 

 face." .... Some have two sets of oblique lines on them, fig. 8, plate 12. Others are 

 waved or curved. 



At p. 150, he sagaciously remarks — " That with the discovery of any more 

 difficult object than that already known, cm improvement in the microscope has 

 soon followed. This was strikingly exemplified in the discovery of the lines in 

 this insect : they were observed accidentally by the late Thomas Carpenter, Esq., 

 of Tottenham." 



Ujion this principle, so well expressed, I venture to make the assertion that 

 until the exclamation markings of this test are admitted to be only a provisional 

 definition, and the beautiful beadeil structure shown in the blank spaces is 

 acknowledged to be a much finer test, no great improvement will be effected. 



