The case of the prismatic prisms
By Earl Henry Driver
A story about Jerry Stoneman, celebrated Dispensing Optician and his faithful receptionist, Bella Lane.
“You see, Bella” said the aforementioned celebrated Dispensing Optician, “when you have a prescription which includes a correction for prisms; if it is expressed as prism in one eye only, then it is quite acceptable to split the prism between the two eyes in order to balance the weight and thickness of the two lenses”
“But what if the prescriber finds out that you have altered his prescription in this way?” asked Bella.
“ I’m sure he would not mind , apart from the fact that he would never know, because of the nature of prismatic corrections.”
“You see; if you have a correction of 2 prism dioptres in the left eye base in, then that may be dispensed as 1 prism dioptre base in, in the left eye and 1 prism dioptre base in, in the right eye and similarly , if you have a correction of 2 prism dioptres base out in the right eye , then that may be dispensed as 1 prism dioptre base out in the right eye and one prism dioptre base out in the left eye.”
“Vertical prism corrections may be split in exactly the same way. e.g. if you have a correction of 2 prism dioptres base up in the right eye; then that may be split as 1 prism dioptre base up in the right eye and 1 prism dioptre base down in the left eye. Similarly, if you have a correction of 2 prism dioptres base down in the right eye, then this may be split as 1 prism dioptre base down in the right eye and 1 prism dioptre base up in the left eye.”
“This up-down does not seem to be exactly the same as the in-out,” observed the faithful and eagle eyed Bella.
“Ah no” said the aforementioned c.D.O. “That is because “in” in the horizontal meridian, appears as “in” in the right eye and in the opposite direction in the left eye, which of course is “in” also.” Whereas, as you will readily appreciate, up and down are “absolutes” i.e. “up” is up and “down” is down regardless of which eye we are talking about, bearing in mind , of course, that up in one eye is the equivalent of down in the other.”
And now dear reader for homework: split the following prismatic correction:
4 prism dioptres base up and out at 135 degrees in the right eye.
(a clue) Your answer should be “y prism dioptres base xx and xxx in the right eye at a degrees, and y prism dioptres base down and out in the left eye at b degrees.” You will immediately know that a and b must be the same, or possibly different.
Answers please by 9 a.m. Monday morning together with any donations to your tutor, who like the left eye is also down and out.
(Apologies to Erle Stanley Gardner, Perry Mason, Della Street et al)