Pedigree Moth-breeding. 23 



of one of the arms a scale is pivottecl ; its free end runs 

 over a pin at the end of the other arm. As the distance 

 between the pivot and the pin is always five times that 

 between the compass-points, the scale is five times the 

 natural size, and can be read off easily to half milli- 

 metres, the pin acting as index. In using these 

 compasses the forefinger slightly presses the scale 

 against both the arm on which its free end rests and the 

 pin over which it runs. When the compass is satis- 

 factorily adjusted an increased pressure is sufficient to 

 clamp the scale, and it can then be read off. I have 

 also tried the glasses from one of the tubes of an opera- 

 glass, with a lengthened interval between them, so as to 

 form a microscope of very long focus, say 18 in. This 

 was fixed on a light rod that carried a millimetre scale, 

 set across its free end at a trifle less than 18 in. from 

 the object-glass. On approaching the scale to within 

 half an inch of any small object, that object and the 

 scale are both in fair focus at once, and they are 

 sufficiently far from the eye to render the error of which 

 I spoke of little or no importance. 



For the accurate measurement of dead moths I have 

 a much better instrument under construction, in which 

 I use a small microscope with cross wires in the short 

 limb of a pentagraph, and use the long limb both for 

 setting the microscope and for reading off the measure- 

 ments. 



The details of the whole procedure are settled thus 

 far only provisionally, as it is reasonable to hope that 

 much has yet to be gained from the past experience of 

 others, and more by the earlier stages of the experiments 

 themselves, so far as they are new to experience. I 

 have considerable hopes that many persons may feel 

 disposed to work with me, for I am sure it will be 

 accepted as an obvious truth by all, whether they may 

 interest themselves in the technical explanations that 

 follow these remarks or not, that sets of broods of 

 pedigree moths, all of whose direct ancestors, male and 

 female, and all of whose uncles and aunts, great uncles 

 and great aunts, and so on for at least six generations, 

 are preserved in convenient trays for reiterated study, 

 would form a collection of first-class importance for 

 hereditary investigation. 



It would be well if in each case of experiment more 



