SPECTACLES

The lenses fitted in a frame constitute the spectacles. It is a common, cheap and easy method of prescribing corrective lenses in patients with refractive errors and presbyopia. Some important aspects of the spectacles
are as follows:

Lens materials
1. Crown glass of refractive index 1.5223 is very commonly used for spectacles. It is ground to the appropriate curvature and then polished to await the final cutting that will enable it to

PARALYSIS OF ACCOMMODATION

(continue  ACCOMMODATION ANOMALIES)
Paralysis of accommodation also known as
cycloplegia refers to complete absence of accommodation.

Causes
1. Drug induced cycloplegia results due to the effect of atropine, homatropine or other parasympatholytic drugs.

2. Internal ophthalmoplegia (paralysis of ciliary muscle and sphincter pupillae) may result from neuritis associated with diphtheria, syphilis,
diabetes, alcoholism, cerebral or meningeal diseases.
 
3. Paralysis of accommodation as a 

Types of lenses

Lenses are of two types: the spherical and cylindrical
(toric or astigmatic).
1. Spherical lenses. Spherical lenses are bounded by two spherical surfaces and are mainly of two types : convex and concave.
(i) Convex lens or plus lens is a converging lens. It may be of biconvex, plano-convex or concavo-convex (meniscus) type .


Identification of a convex lens. (i) The convex lens is thick in the centre and thin at the periphery (ii) An object held close to the lens, appears magnified. (iii) When a convex lens is moved, the object seen through it moves in the opposite direction to the lens.
Uses of convex lens. It is used

What are lenses?

A lens is a transparent refracting medium, bounded by two surfaces which form a part of a sphere (spherical lens) or a cylinder (cylindrical or toric lens).
Cardinal data of a lens
1. Centre of curvature (C) of the spherical lens is the centre of the sphere of which the refracting lens surface is a part.
2. Radius of curvature of the spherical lens is the radius of the sphere of which the refracting
surface is a part.

Cardinal points of a convex lens: optical centre
(O); principal focus (F); centre of curvature (C); and principal
axis (AB).

3. The principal axis (AB) of the lens is the