[photo-3d] prescription "Holmes glasses"

["Abram Klooswyk" <abram.klooswyk@xxxxxx>]

In a thread on Stereoscopes,  Bob Wier  wrote Jan 7, 2001:
 >(...) on my last eye exam, I took along a card and got a 
 >prescription (which included prisms) to allow me to wear 
 >them as a viewer - the major problem is that since they 
 >focus at about 20" (...)

and  Jan 8, 2001:
>the prisms involved make it possible for me to position my 
>eyes more or less normally (ie, somewhat converged) at 20", 
>while still fusing the images. 

The actual prescription is not quite clear from this 
description, but I suppose that it involved lenses with a 
cylindrical component, as used for astigmatism. I also suppose 
they where prescribed for Bob's interpupillary distance.

If this is true, it is not the best possible solution. 
On all regular prescriptions for correcting eye glasses the 
interpupillary distance of the patient is given. Few eye 
doctors would give you a prescription with an interpupillary 
much larger than your own! First you would have to discuss 
stereocard viewing optics. (Don't take for granted that all 
eye doctors know about Holmes stereoscope optics.)

The key feature of "Holmes glasses" is that the separation of 
the optical centers should _not_ be equal your interpupillary, 
but should be at the (infinity) separation of the stereo 
photographs you want to view, with antique cards this is 
about 85 - 90 mm.

Following this principle I have constructed "Holmes glasses" 
from cheap reading glasses, the plastic type sold in various 
places, here even at gas stations. I used the strongest pair 
available, focal length about 25 cm (10''). These are not 
quality lenses but they are satisfactory.

(Because I am myopic I have to use these glasses on top of my 
regular glasses with are made for my interpupillary.)

A maybe superfluous remark on the myth of "prismatic lenses": 
all normal ("spherical") lenses resemble prisms when used 
eccentrical. Oliver Wendell Holmes has cut normal lenses for 
his stereoscope.

Abram Klooswyk