62 MEMO HAND A. 



No. 4. 



The rule presented gave 110 provision for angles exceeding 

 90°; wherefore the supplement of the bi -tangent is used 

 through previous knowledge of that required. 



Decimal No. Logarithm. Tangent. 

 Lights apart 44 inches = >4545 = 657534 or 24 . 26 

 Distance of lens 1U x 2 x 2 



Rule ends = 48'52 

 180— 



Angle of aperture 13108 supplement. 



Another experiment gave 131*06 



Ditto ditto 12624 



Howsoever in theory the proposed plan may be, I am 

 satisfied that even with common care the results in practice 

 must prove very variable, and amounting, in some instances, 

 to differences of four or five degrees ; thus not gaining much 

 upon the quantities ordinarily published by the- makers of 

 objectives. — William Hendry, Surgeon, Hull. 



Diatom -Finder. — I have had a simple little instrument 

 made for me by Messrs. Field and Co., that answers my 

 fullest expectation as a diatom -finder. It consists of the 

 tube of their Arts Society's microscope, six inches in length, 

 with the higher eye-piece fitting into one end, and an object- 

 glass giving 180 and 130 diameters, into the other (the 

 screw also takes Powell and Lealand's object-glasses). This 

 compound body slides into another tube about two inches 

 short, and is checked by a flange from going too far. On 

 the end of this second tube is cemented a thin disc of glass, 

 and over it fits a cap with a thicker disc, forming an ordinary 

 animalcule-cage. When used you remove the cap, place a 

 drop of the fluid to be examined on the end of the tube, re- 

 place the cap so as to flatten out the drop, and hold the body 

 up to the light, adjusting the focus. The contents of the 

 drop by their own gravity may be made to cross and recross 

 the field, simply by moving the body in the hand. The 

 whole fits into a little telescope-case an inch in diameter. 

 ■ — Robert Taylor. 



