Microscopical Researches in High Power Definition. 275 



The full and brilliant display of these minnte spherules is, therefore, 

 attended with no little patient research. (Direct light used only.) 



I wish to call attention to the appearance of a fine full-grown scale 

 of the Podura macrotoma under from 2,000 to 3,000 diameters in 

 three different focal planes. 



Fig. 18 represents the upper focus. 



Fig. 19 represents the middle focal plane. 



Fig. 20. The lowest. 



The Podura macrotoma appears very much to resemble in its scales 

 those of the Petrubius maritimus. Younger scales show single rows of 

 spherules on the ribs, and also between them. 



Fig. 21. A strongly- marked scale, showing the appearance of large 

 spherules.] 



Method of Estimating very Minute Bodies by means of the Micrometer 



Gauge. 



A series of fibres of spun glass is arranged according to thickness, 

 with free ends projecting into the middle of the field of view of the 

 eye-piece, also spider-lines placed across the stop : — 



Thickness of No. 1 accurately measured 1- 1,750th of an inch. 

 No. 2 „ „ 1- 2,000th 



Spider-line No. 3 „ „ 1- 6,000th 



No. 4 „ „ 1- 8,000th 



No. 5 „ „ l-10,000th 



If a positive eye-piece be employed magnifying ten times with an 

 objective of 1-1 0th focal length, the posterior focal image of the glass 

 presents an image magnified 100 times, the total power being 1,000. 

 If, then, No. 1 appears to gauge the size of a magnified object really 

 100 times less — 



(No. 1.) Its size^yyVo x ^=1 -175,000th. 



(No. 5.) The size would be jthro XT^=l-l,000,000th. 



The micrometer guage is not difficult of construction, as the pro- 

 jecting glass fibres can readily be cemented to the stop within the 

 eye- piece. An object is much better compared by bringing it apparently 

 to the free extremity of the fibre, so as to be in juxtaposition. 



Examples of Measurement : — 1. Fibrillce of the H. Janira Moth. 

 l-80,000th to l-90,000th of an inch thick. Black margin of the 

 stem of the fibril about one-fourth of this : thickness of black margin 

 between l-300,000th and l-400,000th. (Fig. 136.) 



2. The delicate Membrane uniting the Minute Spikelets of the Scale of 

 the Gnat. — It may be observed that whenever a membrane is dupli- 

 cated and flattened a dark black line is formed ; the edge is, in fact, 

 semi-cylindrical, and transmits no light. 



