1896 THE 3riCR0SC0PE. 105 



Sir William Logan and liis colleagues, and to tlio furcfnl 

 microscopic investigations of Drs. Dawson and Carpenter 

 must be regarded as an opening of a new era in geologi- 

 cal .science." 



Note On An Optical Rule. 



BY EDWARD M. NELSON, I". R. M. S. 



The rule is made of box, is 20 inclies loiiu and square 

 in section. On one face there is a scale of inches and 

 tenths, and on the opposite side centimetres and milli- 

 metres. On one side of the sides at right angles to these 

 is a scale of dioptrics marked D, and on the remaining 

 side opposite to it is a new scale of powers of marked P. 

 All the scales read from the same end and are ruled on 

 both edges of the rod. This is important, because any 

 two continuous scales can be read and compared together; 

 thus dioptrics can be converted into inches by the in- 

 spection of one edge of the rod, and into centimetres by 

 viewing the other, and vice versa. On tlie opposite face 

 of the rod the scale of powers can be treated in a simibir 

 manner. The following are some examjdes of the use of 

 the rule: — 



(1). When the focus of a lens is measured on the P 

 side of the rule, its magnifying power, wlicu the eye is 

 held at the back principal focus, is indicated. By adding 

 one to this figure the power, Avhen the eye is held close to 

 to the lens, is found: but if one be subtracted from it.tlie 

 enlargement of an image on a screen distant 10 inches 

 from the lens will be enlarged three times, 



(2). When the power of any lens is known, its focal 

 length can be determined by inspection, either in inches 

 or mm. Thus a lens of two-power has a focal length of 

 five inches or 127 mm. 



(3.) The focal lengtli of a diverging lens can be easily 

 found by overpowering it with a converging lens and 

 measuring the power of the combination; this jjower, less 



