People need eyeglasses to correct for two kinds of refractive errors. The eye may make the same focusing error (spherical error) or the magnitude of the error may depend on the orientation of the object being looked at (cylindrical error). The latter problem is called “astigmatism”. Both can be corrected with eyeglasses, but, unfortunately, it is diffJCult to look through a periscope while wearing glasses.
Moreover, it is easy in complex optical systems to correct for spherical errors but not for cylindrical ones. Thus binoculars, microscopes, and periscopes provide a sizeable amount of spherical correction for the user but no correction for astigmatism.
Not only is it not feasible to provide a mechanism for correcting astigmatic errors, but even if it were, it would be very difficult for the user to select for himself the proper magnitude and orientation of the correction. Individuals typically do not even select the optimal setting with a pair of binoculars!
Uncorrected astigmatism results in a significant Joss of visual acuity. If an individual with normal vision can just barely classify a ship at about 10,000 yds, someone with only one diopter of astigmatism must close to about 7,200 yds to see it; with two diopters of astigmatism, he must be about 4,000 yds away, and with four diopters he must come to around 2,000 yds. Yet visual standards allow two diopters of astigmatism., The reason is that it is found in more than 80% of all patients examined. Twenty years ago it was reported that 14% of people in their twenties had it. In a recent survey of 1000 submariners — men constituting a highly selected sample who had already been screened to eliminate large amounts of astigmatism — we found that more than half had a measurable amount. An appreciable percentage of otherwise qualified men are liable to be disqualified from submarine service on this account at a time when the Navy has a problem recruiting enough men.
There is, however, a simple way to introduce corrections for astigmatism into the periscope optics. There is a rather thick plate to which the periscope eyecup is attached. It is a simple matter to fabricate a slot just behind the eyecup large enough to contain an eyeglass lens; the operator’s lens is then mounted in a holder which can be inserted very quickly into the slot in only one way (Fig 1). The lens which would be provided would, of course, correct for both spherical and astigmatic errors. With the lens mounted properly in the holder, the periscope operator’s viewing eye would be perfectly corrected. In practice, each man would have his own lens holder. If he wished, he could have two inserts, one for each eye.
The fact that both spherical and astigmatic errors could be corrected means that visual standards for periscope operators could be relaxed to include those who are otherwise completely qualified.