494 THE OUTGO OF ENERGY 



tion of the observer's eye and the eye to be ex- 

 amined should be relaxed. The observer's eye 

 must be emmetropic ; if it be myopic or hyper- 

 metropic, the defect should be corrected by the 

 proper glass before the subject's myopia can be 

 measured. The correction may be made with 

 spectacles, or with one of the lenses in the disk 

 of the ophthalmoscope. The ophthalmoscope 

 should be placed in the anterior focal plane of 

 the eye examined (13 mm. in front of the cornea 

 of the human eye, 50 mm. in front of the artifi- 

 cial eye). If the observer cannot reach this 

 point, in examining the human eye, the distance 

 between the correcting lens and the anterior 

 principal focus must be subtracted from the 

 focal distance of the correcting lens in order to 

 find the degree of hyperrnetropia, and be added 

 to the focal distance of the correcting lens in 

 order to find the degree of myopia. Viewed 

 from the anterior principal focus, the fundus will 

 be blurred. 



If myopia be present, turn the disk until that 

 concave lens is found which will render clear the 

 image of some one of the vessels l near the border 

 of the optic disk. The rays emerging from the 

 myopic eye are convergent. This lens makes 



1 The error introduced by neglecting the distance between the 

 vessels and the nerve elements of the retina is inconsiderable. 



