48 



BELL. 



ordinary pairs 



Iv^ - m'\=^ 



does not exceed 0.0009 and in a ghost of the 



VM + m'/ 



2nd order this small quantity must be multiplied M'~'^, since there is 

 one free reflection and 2 surfaces are abolished by cementing. The 

 available light in a ghost from a cemented doublet is then 



.0009 X .05 X .90 = .00004 



i.e., such a ghost is 1 1 mg. fainter than its primary, and is barely dis- 

 cernible even under laboratory conditions. 



Fig. 1. 



Taking up now the typical case of a telescope objective with refer- 

 ence to ghosts, we may, in Figure 1, sketch the form of common shape, 

 numbering its surfaces 1, 2, 3, 4. It will give 6 second order reflec- 

 tions from the following combinations of surfaces 



2 - 1 



3 - 1 



4 - 1 

 3-2 

 4-2 

 4-3 



Of these the first four commonly give real images somewhere along 

 the axis, the last two correspond to virtual images in the direction of 

 the incident light, the light being spread to the rear as is usual where a 

 diverging system does the reflecting. The first 3 reflections obviously 

 come to focus relatively near the objective in lenses of ordinary form, 

 and thus merely scatter light of which some actually enters the 

 visible field. The remaining reflection 3-2 is between surfaces opti- 

 cally almost parallel so that after its second reflection the ray is 



