Sunday, March 9, 2008

7.27 Thousand mile eye

Baron Munchausen's friend Adolphus and one of the Southeastern Chinese deity Mazu's (媽祖) two guards, Chien Li Yan (千里眼), both can see objects miles away. These eyes are known as Thousand Mile Eyes. So telescopic vision is a common interest of sorts for both East and West. The real telescope did take a long while to become a reality, though.

Notice in this scene illustrated by Mr Winslow Homer (1836-1910) for Harper's Weekly April 25, 1863 Issue - The Approach of the British Pirate "Alabama" (picture from - the tall gentleman is looking through a Galilean telescope trying to spot the pirate ship.
Galilean? Somehow the invention of the telescope was credited to Galileo of the University of Padua. This is not historically correct because the first documented working telescope was delivered in the last week of September, 1608, by Hans Lippershey from Middelburg in Zeeland to Prince Maurice of Nassau in The Hague. It was not until May, 1609, Galileo was able to quickly fashion an 8X and a 20X telescope to show around publicly. And his reputation grew as a result.

The Galilean telescope is simple to construct. You can put together one, for example, by using a long cardboard mailing tube (5cm in diameter and 110cm long), a +0.75D convex (i.e., plus) lens as the "objective", and a concave (i.e., minus) lens as the "eyepiece/ocular" (power = -6.6 diopter). Cut the lens diameter down to around 5cm to fit in the tube. With some tinkering and adjustment of the focus, you got yourself an almost 9X telescope.

Telescopes are often used in the correction of low vision, obviously not what Galileo et al intended at first. Low vision simply means the patient's central visual field normally with a visual acuity of 20/20 or better is lost, only the para-central or the peripheral retina can now be used for vision. [Note: Patients with retinitis pigmentosa - true Night Blindness - retain a very restricted central visual field and the central vision can be poor as well]. In order to improve vision, for viewing distant objects, the magnification can be achieved through the use of the telescope. And for near tasks, an assortment of magnifiers are available. Magnification is of course what the para-central/peripheral retina needs in order to see well.

The example below is an adjustable 3X spectacle-mounted (clip-on) telescope which also can be used hand-held. It has a plastic casing for low weight. And with a reading cap, it doubled as a reading magnifier for a normal reading distance of around 14-16 inches.

Still kind of bulky, huh? Let's see: the Galilean telescope consists of only two lenses, what if we put one in a spectacle frame, and the other worn as a contact lens? This idea has not escaped the attention of some eye doctors. With the advent of high-index spectacle lenses and high-power soft contact lenses, this is now readily achievable. For those who are interested, this is how it is done:

Let's say you need an around 1.7X telescope, use a contact lens of -50D (i.e., -20mm focal length) and a spectacle lens of +30D (i.e., 33.3mm focal length), then the vertex distance is -20+33.3 = 13.3mm. The vertex distance is the distance between the spectacle lens and the cornea (normally around 13.5mm), and in the present case, give and take because of the lens thickness. And the magnification is 50/30 = 1.66X.

By lengthening the vertex distance just a bit, the power of the lenses can be reduced. The added advantage is lower power lenses are easier to make correctly. So, for a vertex distance of 15mm, the following combinations are possible (lens power in diopter):

Objective Ocular Magnification
+15.37 -20 1.3
+20.7 -30 1.45
+25 -40 1.6
+30 -50 1.75
+31.62 -60 1.9

You can play with the numbers and construct your own table.

Lens weight is a major concern - heavy lenses become mis-aligned quickly and the "Contact-lens Telescope" can no longer achieve the desired magnification. High-index light-weight spectacle lenses therefore must be used. RGP corneal contacts are not stable enough, scleral/macro lenses maybe a better alternative [to be verified]. Large-diameter soft lenses usually work out well; although the power must be as accurate as possible.

As usual, not all patients are suitable candidates.

Lest we forget, mini-telescopes also can be implanted in the center of the corneas - to restore vision of patients with retinal damages, such as AMD. A recent clinical trial has proved efficacious. However, the FDA eventually did not approve the device because of the concern of corneal endothelial cell loss - which occurred at a higher rate than the normal age-related loss.

Back to the drawing board.