12 Transactions of the Society. 



So that with the more favourable blue ray the smallest interval 

 visible among contiguous bright disks or lines is about one hundred 

 thousandth of an inch, and that only with the largest aperture. 

 Such is the belief disseminated. 



About ten years ago I requested Messrs. Beck to make for me 

 an "iris diaphragm" with "adapters" on each side. By this inge- 

 nious contrivance, screwed between an objective and the body, the 

 angular aperture could be instantly reduced at will. 



It seems, on the face of it, not a little surprising, considering 

 this famous optical law, that the visibility of lines of great minute- 

 ness is very little afi'ected by great reduction of objective aperture, 

 by means of this instrument, or by using low- angled objectives of 

 sufficient power and excellence of manufacture. Apparently this 

 is another failure of the celebrated law, as roundly stated and 

 generally received. 



It will be convenient to explain here two practical points : — 



A. — The method used in finding the diameter of the spider 



lines enclosed within the micrometer. 

 B. — The method employed in measuring the absolute reduction 



of the object in miniature. 



A. — The Rev. ]\tr. Dallinger has given us his beautiful measure- 

 ments of the flagella in monads, by drawing an equivalent line with 

 a very hard fine pencil on white paper, by means of the camera 

 lucida. By this process he, after a great many observations, 

 determined its diameter to be less than the two hundred thousandth 

 of an inch. 



The plan I adopted was by finding divisions on glass placed in 

 the focus of the eye-piece which appeared perfectly coincident in 

 diameter with the observed spider line; and then substituting a 

 scale of a hundred thousandth of a metre, a most careful measure- 

 ment was made of the apparent size of the diamond cut. The 

 process was much facilitated by altering the length of the draw 

 tube, and changing the objective until the most acceptable result 

 was arrived at. I am indebted to Mr. Beck for the use of an 

 exquisite scale of this kind, as also for the loan of ^^th objectives, 

 dry and immersion, which latter has reduced the miniature to the 

 extraordinary minuteness and precision of definition, at seven 

 inches, of one hundred and forty times less than the object. 



On examining spider threads, gathered after recent spinning, 

 with Powell and Lealand's best I dry, and measuring them with 

 the spider-line micrometer inserted in the body, I was charmed 

 with perceiving the characteristic brilliant central band, due to a 

 minute cylindrical lens of great beauty, and perfection of definition : 

 and searching for threads lying flat and in clcse contact, I ibund 

 some consisted of four cylinders in contact, showing four bright 



