G. WEST ROYSTON-PIGOTT ON OBJECT GLASSES. 



37 



From this it appears that an exact sixteenth should produce an 

 image precisely 158 times as large when the object is exactly 

 ten inches from the field of the eye-lens at the stop of the eye- 

 piece. 



For practical purposes, therefore, an eye-lens magnifying ten 

 times would enlarge the object in this case 1,580 times. Now a C 

 eye-piece of Powell and Lealand is just equivalent to an one-inch 

 lens ; therefore, when these makers announce their sixteenth to 

 magnify 1,600 times with a C eye-piece, this objective is nearly the 

 yi-g- of an inch focal length within a small decimal. 



The magnifying power employed at any moment is often so great 

 a desideratum, and yet so unattainable (when one is closely engaged 

 in some delicate investigation, and using a variety of objectives), 

 without great loss of time, that the following observations upon a 

 simple method exhibited at the meeting of the Fellows of the Royal 

 Microscopical Society last month, may, it is hoped, prove acceptable. 

 Having met with many persons and some opticians who experienced 

 a difficulty in understanding the reason of the thing, I trust that 

 the preceding remarks will clear the difficulty away. 



If we settle it as an axiom for very high powers, such as the -J-th 

 and yV^h, that at ten inches distance of the stop of an eye-piece, 

 without the field -glass, the enlargement of thousandths on the stage 

 will give the focal length simply by dividing ten by the amplifica- 

 tion increased by two, then it is evident that by using a single lens of 

 one-inch focal length magnifying ten times, if we count how many 

 hundredths of an inch in the stop correspond to a hundredth on 

 the stage micrometer, ten times that amount with an inch or C eye- 

 piece is the magnifying power. Now replace the field-lens (usually 

 of 3-ihch focus) for an eye-piece of 2-inch focal length, having an 

 eye-lens of one inch, the magnifying power will be reduced con- 

 siderably in the proportion shewn by the new reading. Whatever 

 object glass is now used, and whatev-er length of tube happens to 

 be in use, so long as the eye-lens is 1-inch focal length, teji times 

 the apparent amplification of the stage micrometer will give the 

 power under employment. 



I keep an eye-piece (two-inch) with one-inch, eye lens, armed 

 with a glass micrometer, ready for use. Every microscopist 

 should demand that the optician mark the focal length of each of 

 his eye-pieces. Powell and Lealand's C eye-piece is exactly one- 

 inch focal length ; and at the usual distance of ten inches the power 

 of any object glass with it is at once found by multiplying the re- 



