Abstract: A magnetic tape cassette holding a wound magnetic tape comprising a transparent base film and a magnetic layer having a thickness of 5.mu. or less coated on the base film comprises a hole facing a light source for detecting the tape position, a hole facing a photosenser provided so as to form a light path passing to the hole for light source through which the magnetic tape is passed and a light-shielding filter having a light transmittance in the range between 0.5 and 10 percent placed in the light path.

Abstract: A magnetic recording tape comprises a polyethyleneterephthalate base film coated with a magnetic layer which has an index H given by the equation:1.5>19.00-0.406 d.sub.B -0.016 H-0.553 d.sub.Td.sub.T .ltoreq.18(.mu.m)wherein the reference d.sub.T (.mu.m) designates a total thickness of the magnetic recording tape; d.sub.B (.mu.m) designates a thickness of the base film and H designates a sum of numbers of interference rings (H.sub.1, H.sub.2, H.sub.3 . . . )per 1 mm.sup.2.

Abstract: A magnetic recording tape comprising a polyethylene terephthalate substrate and a magnetic material-containing layer closely adhered onto the substrate, which tape exhibits a reduced envelope variation. The polyethylene terephthalate substrate satisfies the formula:0.051H.sub.1 -0.59A+0.14 .DELTA.n+0.0015 (H.sub.2 -66.67).sup.2 +50.92.ltoreq.20wherein A is the average particle size of the polyethylene terephthalate crystals as determined from the half height width by X-ray diffractometry, and H.sub.1 is the average number of projections present in a unit area of 1 mm.sup.2 on the surface of the substrate, which projections have a height falling within the range of from 0.27 to 0.53 micron, H.sub.2 is the average number of projections similar to H.sub.1 above but having a height falling within the range of from 0.54 to 0.80 micron, rather than 0.27 to 0.53 micron, and .DELTA.n is the birefringence of the substrate.